So you own a plain-vanilla, nonhybrid, American-made gas guzzler and can't afford (or can't wait for) a hybrid. Now what? 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%. The bottom line? 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.