Don't let them trip you up: Get an engine flush only if you've been driving the car for several years and notice a buildup of greasy material under the oil cap. If a mechanic says you need a flush because your oil is "dirty," he's lying.
Fuel injectors should be cleaned, but usually not until they've logged a lot of miles (think 100,000 or more). If the mechanic says the ones on your 20,000-mile car are filthy, he's full of it. However, if you do suspect that there might be a problem with your fuel injectors, play it safe and add fuel-injector cleaner to your gas tank during your next fill-up. Otherwise, if you buy good-quality gas and have a fairly new car, your injectors probably don't require cleaning.
They do not work. Period. To save gas, accelerate gradually, avoid unnecessary braking, empty your trunk of junk, and learn how to coast effectively.
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Auto-Transmission FlushGeorge Clerk
Don't even think about it until your odometer has reached 60,000 miles, since most cars have filters that keep transmission fluid flowing freely.