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Fact or Fiction? 8 Myths About Your Car

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By Lou Carlozo, Green Dad columnist for dealnews

The Kelley Blue Book folks have just named their Top 10 Green Cars for the 2011 calendar year, a seeming boon for drivers seeking eco-friendly automotive alternatives. But if you're anything like me, you might think it's not easy being green, especially when the path to environmental friendliness runs straight through your wallet like a Hummer on steroids. A bare-bones 2012 Ford Focus — number 10 on the Kelley list — runs $16,270. And that's cheap compared to the fourth-ranked Lexus CT 200h, weighing in at $29,120, also without options. Ugh. To add insult to injury, any new car depreciates the second you drive it off the lot.

So for those of us who don't have five figures to spend on a gleaming set of wheels, take heart: The greenest car may well be the one you already have, provided you take steps to make it more fuel-efficient and environmentally sound. This is especially so because you're going need those five figures to pay for skyrocketing gasoline this summer (you can also follow my tips on ways to save money on gas). This week, Green Dad sorts the myths from facts when it comes to making your machine run in harmony with the planet.

Fact: Driving slower saves you gas. Like millions of Americans, I listen to the "Car Talk" guys on National Public Radio mostly because they make me laugh. But they also dispense great auto advice, and they stress that slowing down makes a big difference: For every 1,000 miles you drive (figuring gas at $2.50 a gallon and 25 MPG fuel efficiency), you'll save as much as $15 driving 10 mph slower. Of course, with gas nearly double that price now, the savings should double as well. Something to think about if you're a speed demon.

Myth: Replacing the air filter on your car improves mileage. Every time I go for a quickie oil change, the tech comes out with an air filter that looks like dirt, which shames me into coughing up to replace it. But if your motivation is better gas mileage, consider skipping the change. The U.S. Department of Energy reports that an air filter change will mostly help your acceleration, not your mileage — though if you drive an older car with a carbureted engine, it may improve fuel economy 2% to 6%.

Fact: Stepping on the brakes wastes gas. This one also comes courtesy of Click and Clack on "Car Talk." "Every time you use the brakes, you're wasting the 'acceleration' you've already used. Instead of moving your car, that energy is being transformed into steaming hot brake pads, they say. Instead, learn to anticipate stops, and gently accelerate your car from a standing stop. For more of those great "Car Talk" tips, click here.

Myth: Gas-saving additives can improve fuel economy by 20% or more. What's the difference between motor oil and snake oil? Not much, if you're weighing the claims of slick entrepreneurs who know the timing's right for selling you magical gas-saving potions. To be sure, some long-standing products with modest claims (such as STP Gas Treatment) have vigorous supporters, but they only claim to boost mileage about 10%. But as the Federal Trade Commission warns, "The Environmental Protection Agency (EPA) has evaluated or tested more than 100 alleged gas-saving devices and has not found any product that significantly improves gas mileage. In fact, some 'gas-saving' products may damage a car's engine or cause substantial increases in exhaust emissions.

Myth: New "low-resistance" tires significantly improve gas mileage. Unless you're driving bald or severely under-inflated tires, the hype about low-rolling resistance tires adds up to minimal gas savings. As this story by USA Today reports, various brands of these newfangled tires create gas savings of only 1% to 3%, even if rolling resistance is cut by 25% or more. Whether that saves wear and tear on your car is another story, but the mileage improvements won't even pay for a spare tire over the course of 10 tanks of gas.

Fact: You can realize dramatic mileage improvement by replacing your oxygen sensor. This falls under the category of tuning up your car, always a good idea if you want to see gas mileage gains in the 4% range. As a function of any proper tune-up, ask your mechanic to look at the oxygen sensor; if it's not working properly, a simple repair to this part (commonly costing between $50 and $120) could boost your mileage by as much as 40%, the U.S. Department of Energy states. Note that if you make this fix, it may take a few weeks for you to realize the improved mileage as your engine's fuel-air ratio adjusts.

Myth: Topping off the gas tank is a good idea. I know, I know: You've finally found the one pump between here and Logansport, Ind., that has affordable gas, so why not squeeze every last drop into your tank? While that sounds sensible at first blush, here's the problem: Gas expands in warmer weather, which means topped off gas will likely wind up spilling out your tank. As if you needed more incentive, check out this warning from the U.S. Environmental Protection Agency: "Topping off the gas tank can result in your paying for gasoline that is fed back into the station's tanks because your gas tank is full." So unless you're in the mood to pay the folks at Exxon or BP a little more for your fuel, stop topping off your tank.

Myth: High-octane gasoline improves your mileage. I'm amazed to see that countless drivers still fall for this marketing gimmick, despite abundant evidence that high-octane gas is a waste of money. Props to the folks at Bankrate.com for pointing this out, along with other gas-saving tips you can read here. Bottom line: Unless your car specifically requires premium, skip it and go for low-octane fuel.

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Lou Carlozo is dealnews' new Green Dad columnist. He was most recently the managing editor of WalletPop.com and, before that, a veteran columnist at the Chicago Tribune. Follow him on Twitter— @LouCarlozo63. You can also sign up for an email alert for all dealnews features.


Photo Credit: TheBusyBrain via Flickr.

DealNews may be compensated by companies mentioned in this article. Please note that, although prices sometimes fluctuate or expire unexpectedly, all products and deals mentioned in this feature were available at the lowest total price we could find at the time of publication (unless otherwise specified).
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22 comments
offdwall
Actually I remember those days. My mom gave me 50 cents and I'd buy a gallon for the mower and I always had change left over. My brother just sold his 55 with a 427 and bought a Viper (he's getting old). I remember going to the drags with my brother's car club and they would stick a dollar on the dash and tell you that when the light turned green, if you could grab it, you could have it! The g forces were killer and I lost more than I won. He moved from a 57 Chev into a Mustang in the 60's when 12's were fast, then graduated to one of the first floppers soon after (64 Falcon with a Blown 426 Hemi)!

Have you been to the NHRA Museum? They have a demo of what a Top Fuel cars burns in 1 cylinder during it's 4 second run. Now that's 1/4mpg!
MGNYC
Buried in the good advice that stepping on the brakes wastes gas is this incorrect suggestion: "gently accelerate your car from a standing stop." My understanding is that for optimum fuel efficiency you should accelerate briskly to your target speed and then hold it at that speed. This not only maximizes your own fuel efficiency, you get a double bonus by helping move more cars through the same green light. (Obviously this doesn't apply in stop and go traffic or when going stop sign to stop sign, since there's no opportunity to hold a target speed)
srb015
As soon as I read "your air filter does not improve gas mileage..." I stopped reading...

Simple reason why is that I have not changed my air filter for about 8 months, and it got so dirty that my V6 engine went from 21mpg usage to 14mpg... as soon as I switched clean air filter I got back into 20-21mpg... LITERALY!
mikiem
You were born too late!
I remember paying less than 50 cents a gallon for what amounted to almost aircraft fuel, running a Chevy 427 bumped up to 454 with ~650 horses -- needed the octane too at 13.5:1 compression. You could set a beer can in each throat of the 2ndary half of the 4 bbl carb -- open it up & you could actually see the gas gauge dropping. & that was nothing next to friends with superchargers sticking up through the hood -- used to swipe them from buses when the driver was inside whatever diner having lunch. 'Course thinking back we were all stupid as the chasis, tires, brakes etc. at the time could not handle those speeds on those roads, anywhere near safely.
mikiem
If you really want to boost performance, for a *while*, add mothballs.
[BTW I meant the ones you buy at the grocery, though I imagine if you're off collecting that other stuff you might take a magnifying glass & tweezers & try the real thing too.]
mikiem
FWIW I agree with "At low RPMs, in an engine tuned for regular gasoline, it'll make no difference.", but IMHO no one should discount the fact that cheaper often means cheaper.... There's a difference when you go to the grocery store, where the quality of what's inside the can varies between the cheapest & the most expensive, & there's the same difference at the pumps. It's marketing 101. If you want the customer to buy the higher priced product, make sure they notice the difference. If a customer [buying reg] complains, well they should have bought the good stuff to begin with.
mikiem
Purely FWIW & no offense intended, the guys in the shop love the engineers & PhDs who happen by -- not saying this about anyone in particular, but as soon as someone starts off with I'm an engineer or PhD or whatever, mechanics chuckle -- that's NOT a negative critique, but more a heads up to the engineers etc. out there [yes, chances are techs really are laughing behind your backs].

Now, to say "Fuel economy is not "an RPM thing"" is going against the laws of physics -- energy is not created, but released &/or converted, so for a motor of any sort to go faster, it has to consume more energy, whether you're talking about the electric motor powering your desk fan, your electric car, or an internal combustion engine. Yes there are a huge number of variables that come into play when you calaculate overal mpg for any vehicle, but physics still works no matter what -- to increase the RPM of a gas engne you have to supply more fuel, while one way to lower the RPM is to supply less, i.e. take your foot off the gas pedal.

This is almost right, but doesn't go far enough: "Each engine has a characteristic point of maximum efficiency ... where it will produce the greatest power output for the least amount of fuel... tune their transmissions ... engine is operating as efficiently as possible." Trans *tuning* for mpg is more about when to kick in/out of overdrive, lock up the converter etc. -- other *adjustable* trans variables have more to do with performance, e.g. masking the often negative effects of having a higher mpg engine so that my Honda Civic drives more like an old V8 Chevy than a Pinto. Intake & exhaust OTOH, basically the engine breathing has a huge effect on any *sweet spot*. That's how/why re-programed aftermarket chips can provide cars with performance boosts -- they change the way the engine breaths. That's why people use [exhaust] expansion chambers on their bikes, why headers can make such a huge difference, why racers port/polish their cylinder heads etc... Where you put that sweet spot matters -- an engine tuned to run best at full throttle can be nasty at low RPM, & may not even idle. An engine that purrs like a kitten at the stop light can be too anemic when you're pulling out in front of a semi going 90 mph. So they compromise, & use electonics to broaden that sweet spot, e.g. Wikipedia on variable valve timing http://goo.gl/ZVnDp .

Again, no offense but this is a bit unrealistic: "... check your owner's manual, and use the octane level recommended by the manufacturer for best performance." Gas varies by locale, & often by station. Owners manuals are written in large part by marketing -- often the definition of extreme use in those books is everybody's everyday use. Pay attention to mpg & performance & use what works best for you... simple.

"Modern O2 sensors last virtually forever (most will easily last the lifetime of the engine), and they either work perfectly, or they don't work at all." Not exactly... In a lab or on a test bench maybe, but in real life the biz end doesn't live in a nice, pristine environment. They can often OTOH be cleaned, & either cleaning or replacement can make a noticable difference in how well a car performs. Also, what do you consider the lifetime of an engine -- with a decently maintained Toyota, Nissan, or Mitsu for instance 150 -200k is about the range to start thinking about a rebuild, more of a maintenance thing for the next 150k or so. ;-)

A quick final note... "they will give you a check engine light or some other form of warning if you need a replacement" Do Not Rely On or 100% trust check engine lights or the electronics that run them. That's just drinking the KoolAid the marketing types give you. A car or truck can be one of the most hostile environments electronics will ever see. Add in you're often talking small, trace voltages carried over yards of wire passing through several un-sealed, very often corroded, push/snap together, crimped on connectors. EVERY dealer svc dept. can tell you horror stories. Get to know one of their older techs & they might even tell you about *unofficial*, official fixes that involve disabling functions, lights, etc.

I really do not mean to offend by correcting a couple things above... if it matters I did my 1st custom paint [on my bike] 50 years ago & never looked back, doing body work at 13, becoming one of those street racers like in the movies [Never looked that good, but I had a few classics like a 64 Goat, 65 Mustang Conv., the Chry. version of the Nascar Charger etc]. A motor-head who loves cars enough I went from working on cars while in school to running a few shops & svc depts rather than using what I learned as an Org. Chem major [w/4.0]. And for the green folks reading this, I wasn't lying about the Honda Civic -- I'm reformed. :-)
offdwall
I can't believe that all of these "engineers" don't have something better to do. OK, so to get the best gas mileage I should fill my car with diesel, sit in my driveway, with the brakes on.

or

I can continue to drive my Dodge Hemi at light speed, filling up every 200 miles, while snickering at the people in the Prius crowd.

Fossil fuels are not the answer, but as long as they are around no one has the incentive to make the next generation fuel. By using up my gas as quickly as possible I am helping to save the planet!
pretol
Not everyone has a Back-to-the-Future DeLorean that can break all laws of physics like you do!!!

And not all countries run by some made up laws that every citizen makes up for him/herself!!!

And not everyone can be so bold as to then proceed to call the made up laws of physics and judicial laws as "true science"... Some of us, already know the answers to these questions, and we don't need you to make your own data to support the existence of your dream world.

I have no idea what this has to do with "global warming morons", since the concepts of mechanical efficiency existed WAY before the concept of global warming. So, I don't know why you're trying to spread propaganda in a rather technical forum.
pretol
1. RPM is not in direct relationship with efficiency. Gasoline burns over a bell curve. If RPM's too low, the gasoline burns too quickly, and the rest of the piston cycle is wasted by unnecessary friction with no production. If RPM's are too fast, then the bell curve starts falling off the piston cycle, and is actually burnt in the exhaust.
LOW RPMs does NOT EQUAL efficiency. It's the most common misconception.

2. Regenerative breaking is VERY INEFFICIENT. It's better than nothing, but you'll probably only get 1/3 of the energy back. But your suggestion that stepping on the brakes is a good practice is plain WRONG. One should minimize brake usage in a regenerative braking vehicle just the same as with any other vehicle.

3. Higher octane, again, has to do with the bell curve of the gasoline burn. Bell curve has the same area under it, regardless of what gasoline you use. The difference is that higher octane gas has a narrower peak (fewer lighter and heavier HC's). The gasoline is actually lit up before the valves are closed, since it takes a while for it to light up, and you want the total burn of the gasoline to happen during the whole power stroke (any fuel burnt outside the power stroke is obviously wasted). Theoretically you can actually get worse MPG by putting better gas, since it'll ignite faster (not to be confused with pre-ignition), and some of the burn might happen before the power stroke even begins - theoretical though. Practically, it makes LITTLE TO NO difference. Higher octane fuel is good for staying efficient at higher RPMS, only in engines that have the appropriate timing. At low RPMs, in an engine tuned for regular gasoline, it'll make no difference.

A good counter-example is a diesel engine. According to you, diesels should get crappy gas mileage (diesel is a very low octane fuel). Yet, we all know diesels get better efficiency (even given the higher energy density of the diesel vs gasoline). According to you, people should put gasoline in their diesel motors, and somehow "magically" get better mpg.

You have such disregard for simple laws of physics, are you sure you're an engineer?
broadcastdave
You can double your gas mileage by adding 3 drops of genuine unicorn sperm.
hischan
Just a month ago, you were touting the benefits of STP Gas Treatment in regards to fuel efficiency...(http://dealnews.com/...ve-Money-on-Gas/442628.html). Perhaps, Dealnews should try to verify STP's claims...I'm sure a lot of people would like to have a definitive answer.
Dan de Grandpre (DealNews)
Fortunately for Dave, It is completely LEGAL to claim that things are illegal when they are in fact not. Hawaii has Dave's 10 mph law, according to Wikipedia, but no other state has that limit that's mentioned. (Some other states have different minimums.) See "Speed limits in the United States" on Wikipedia.
bgraskow
Yeah, and if you're going down a steep hill at 65 MPH with a strong tail wind, I'm sure you'll get better mileage than if you're going up a hill and pulling a trailer at 25 MPH too.
dave.dynamo
Sorry, but theory doesn't replace reality. Every vehicle I have ever driven gets better fuel economy at 65MPH than at 25MPH. Why? Roads set for 25MPH have stop signs and stop lights. Highways don't. Higher speed can and often does mean better economy.
dave.dynamo
Fact: Driving slower saves you gas. FALSE!!!!! As pointed out by jbrsci, this ignorant, often ILLEGAL advice is irresponsible. Stop listening to "environmentalists" who make these claims and start thinking. My vehilce gets it's best mileage right at 65MPH. I get about 15% less at 55MPH, and about 10% less at 75MPH. So given the choice between driving 10 under or 10 over, 10 over saves me more, and is far less likely to cause a collision. Most drivers on the highway tend to average 10MPH over the limit, and a speed DIFFERENTIAL of 20MPH (75 minus 55) is very likely to cause severe accidents.

Further, driving 10MPH under the limit is ILLEGAL everywhere! Doing so is called obstructing traffic, and not only does it cost more money in reduced fuel economy, it also causes accidents, annoys other drivers leading to them being less safe, and is simply rude.

Stick to true science, and stop repeating what the global warming morons (read this as shift jobs from the US to other countries) are trying to force feed down people's thoughts.
ethanvyce
one note about "green" cars to consider: keeping your current car (or buying a used) is, in general, way greener than any new car, even a hybrid or electric if you consider the energy/ material needed to produce the car.
mikiem
+1
In fact, as most bicyclists &/or anyone who's timed the lights on their way to/from work will tell you, it's best not to deviate at all from whatever set speed -- any increase takes extra energy -- any decrease means you'll have to spend extra energy reaching cruising speed again. Braking (vs. coasting) is worse yet because it takes energy to counteract momentum, & whether you recover some of that or not, it's still more total energy expended.
mikiem
If it helps at all...

RE: speed... Obviously bottom line with mpg is how much fuel is fed to the engine... the faster a regular internal combustion engine runs [i.e. the more you step on the gas, allowing more fuel to enter] the more gas it'll burn. Less rpm = less fuel used = more mpg. Beyond that simple logic a huge number of factors comes into play, including road surface [i.e. how much energy is actually used vs. wasted], atmospheric conditions [e.g. cooler/denser air is better], prevailing winds, road incline, aerodynamics, & varying from one brand model to the next, the sweet spot designed into the motor itself -- the amount of air/fuel entering the cylinders, how fast it can enter, & the route the intake & exhaust separately follow as the engine breaths is optimized for a certain range, e.g. race engines tuned for acceleration & top end idle poorly. Modern electronics control systems have broadened this sweet spot considerably, but it's still there, as intake & exhaust air paths remain constant & engineers fine tune their product. The end result is that at certain speeds your car just runs more efficiently than at others, & you may or may not see increased mpg because of it, e.g. your car's engine may be tuned for 65 mph because at that speed its aerodynamics &/or gear ratios etc. don't work so well & any boost just makes up for it. ;-)

RE: Fuel Octane... standard internal combustion engines work by compressing an air/fuel mixture, then exploding it. The more compression you have, the more powerful the explosion -- "back in the day" 13:1 compression wasn't rare for performance motors, then since lead was the primary octane booster, the un-leaded gas necessary for catalytic converters brought that down to the neighborhood of 8.5:1. Octane in a nutshell means how much pressure that air/fuel mix will tolerate before it explodes on it's own, rather than waiting for the sparkplug to ignite it [diesels BTW don't use sparkplugs & just rely on very high compression]. So, as long as the mixture of gasoline & air doesn't blow up on its own, you're good to go & increasing the octane beyond that is meaningless. *BUT* buying a higher grade of gas can sometimes make a noticeable difference because of the higher quality of the ingredients, because the cheapest grade may not *exactly* meet specs, because additives can make a big difference with the alcohol/gasoline blends we run nowadays & their consequences to your car's fuel system. [I like to joke that when Henry Ford made the 1st Model T, alcohol was a lot more plentiful than gasoline, and he chose gasoline for a reason.]
bgraskow
As someone who has a PhD in mechanical engineering specializing in engines and combustion, here are a few corrections on the previous post, as well as one clarification from the original article.

1. Fuel economy is not "an RPM thing" or simply a function of torque. Engine fuel consumption is determined by the power delivered- which is the product of torque and engine speed (RPMs). The example given above of a car needing more fuel to go 35 MPH at 2600 RPM vs one going 65 MPH at 2300 RPM is wrong, the situation is signifcantly more complex than that. The power needed to push a car through the (considerably higher) wind resistance at 65 MPH will be greater than what is needed at 35 MPH, regardless of the engine RPMs. Each engine has a characteristic point of maximum efficiency in terms of engine speed and throttle setting where it will produce the greatest power output for the least amount of fuel consumed, and automakers will tune their transmissions to make sure that when running at the expected cruising speed of 65 mph, the engine is operating as efficiently as possible.

However, drag (the force of wind resistance) increases as the cube of velocity. For example, if your engine needs to produce 10 HP to move your car down the highway at 35 MPH, then 65 MPH would require about 64 HP, a 6.4-fold increase in the power requirement. The modest gains made by engine/transmission optimization cannot overcome this fundamental law of physics. Ever puzzle over the old Seinfeld gripe about "why don't airplanes always fly as fast as they can?" - it's because going faster increases drag (air resistance) which costs more in fuel and therefore in money.

Bottom line: driving more slowly will cost less gas, but it also costs you time. All things considered, you're probably best off driving on the freeway at around 60 MPH.

2. Hybrid drivers should not use their brakes any more than they have to. Although hybrids can capture some of the braking force and store it in the batteries, then re-use this during acceleration, the fundamental laws of thermodynamics assure us that this cannot be done with 100% efficiency. In fact, a typical number for efficiency of capture and re-use of regenerative braking on a current-generation Prius is often quoted as being somewhere around 30%. That means that for every unit of momentum that you waste by applying your hybrid's brakes, you will permanently lose 70%. Better than the 100% lost in a typical auto, but definitely not a reason to use your brakes unnecessarily. So hybrid drivers, don't use your brakes any more than necessary!

3. Higher octane fuel has a higher resistance to pre-ignition (knock) which is highly detrimental to your engine. Higher octane fuels do not, in general, contain a greater amount of energy than those with lower octane; they simply are less resistant to pre-ignition.Engines with higher compression are more prone to pre-ignition, and therefore need higher octane fuel to operate at their peak efficiency. These engines typically contain knock sensors, and if a lower octane fuel is used, the engine ignition will be shifted away from the optimum point to prevent knock, reducing efficiency. Using a higher octane fuel in such a car would definitely improve fuel economy.

In engines with lower compression ratios, the tendency for pre-ignition is much lower (assuming that the engine is in good working condition and does not have excessive deposits), and its ignition can be timed optimally even on regular unleaded fuel. Such an engine would see absolutely no improvement in fuel economy, not even one which is too small to be measured.

Bottom line: check your owner's manual, and use the octane level recommended by the manufacturer for best performance.

4. Don't replace your oxygen sensor unless it is warranted. While it is true that a bad O2 (oxygen) sensor can cause a huge hit in fuel economy, replacing an old (but working) O2 sensor will do absolutely nothing for you. Modern O2 sensors last virtually forever (most will easily last the lifetime of the engine), and they either work perfectly, or they don't work at all. Furthermore, most engine computers can detect when an O2 sensor goes bad and they will give you a check engine light or some other form of warning if you need a replacement. I'm sure most auto repair shops will gladly replace your O2 sensor if you ask, but in most cases, you would just be wasting your money.
ryanlionrah
In reference to jbrsci's second point, while hybrids do have a system in place to recapture energy from braking, they should still try to stop as little as possible. It is not "actually good to brake often". Not braking at all will still produce the highest mileage, as it is impossible to convert energy from one form to another (mechanical to electrical to mechanical) without losing some of said energy. The hybrid system merely reduces, not eliminates, the amount of energy lost.
jbrsci
From working as a mechanical engineer in Tier 2 automotive, I'd like to clarify some of the above points. While all of them are generally correct, a few caveats should be made especially for people that drive hybrids, high-compression or sporty cars, or any car made in the past 6 years.

1. Driving slower will save you gas only if that means the torque on your engine is reduced. Car transmissions today are tuned so that they are most fuel efficient at about 65 MPH, in their highest gear or in overdrive. If you slow down to 35, the air and road drag on your car will reduce, but you could very likely require more continual torque delivery at that speed in your car due to it switching to a lower gear for better potential acceleration at that speed. It's an RPM thing, and something your car transmission is designed to optimize as best it can. If your car hums at 2300 RPM while cruising at 65 MPH, but it needs 2600 RPM to drive 35 MPH, then 35 MPH probably costs you more gas. Now, if you were to speed up from that 65 MPH to 80, that car that is tuned to 65 will definitely have reduced fuel economy at 80. And you'll see the RPMs increase when you increase to 80. That's the cause.

2. Stepping on the brakes wastes forward momentum, which can only be regained by forward acceleration. In a normal car, forward acceleration can only be created by burning fuel or going down a hill. In a hybrid, things are different. It's actually good to brake often. When you brake, regenerative motors capture the braking deceleration and store it in batteries. When you take off again, that energy is used once more in preference to the gas engine. That's why hybrids usually get better mileage in stop-go city driving than on the highway, which is opposite of normal cars. So hybrid drivers, don't be afraid to stop a lot!

3. Higher octane gas does improve fuel economy. In a basic passenger car engine, this improvement is small so it may not be noticed amid normal fluctuations in driving. But if you drive a sportier car, say a Lexus IS 350 or a Nissan Z, those engines prefer higher octane and will burn noticeably more efficient with high octane fuel. In fact, depending on the gas prices you see, you can even save money by spending the extra dime per gallon in those cars. That's why their manuals indicate 89 or 92/93 octane fuel--they really do need it to be efficient.

All of the other statements are basically dead on. Very good summary for the masses!