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. To find the latest deals on car gear, check our automotive section on dealnews, or set up an alert on our new free iPhone app. 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).