So you own a plain-vanilla, nonhybrid, American-made gas guzzler and can't afford a hybrid. Now what? If you can boost your gas mileage from 20 to 24 m.p.g., your old heap will put 200 fewer pounds of CO2 into the atmosphere each year. Just giving your engine a tune-up can improve gas mileage 4% and often much more. Replacing a clogged air filter can boost efficiency 10%. And keeping tires properly inflated can improve gas mileage more than 3%.