The kind of gasoline car owners put into their cars can affect not only performance, but also how long the engine will last before it needs internal repairs.

If the car is hard to start, runs roughly, or stumbles on acceleration, Better Homes and Gardens magazine suggests trying a couple of tanks of a different gasoline before heading off to an expensive mechanic.Virtually every car sold since the 1975 model year requires unleaded gasoline. That's because lead, used since 1922 as an inexpensive way to boost a gasoline's octane rating, also damages emission control components.

Octane simply indicates how fast gasoline burns. Most cars require 87-octane unleaded fuel. Check the car manual. Fuel with too little octane can damage a car. Fuel with too much octane provides no extra benefit.

Tip: Although leaded gas is slightly cheaper than unleaded, don't risk damaging a car for a few cents a gallon. However, little harm is done if unleaded gas is mistakenly pumped into an old leaded-gas car.

An engine creates power by burning a mixture of gasoline and air inside the cylinder. The power comes from a controlled explosion of the mixture pushing the piston down. If the mixture burns too quickly or ignites prematurely, a sound rather like rattling dice - called "knocking" or "pinging" will be heard. Unchecked, this can damage the pistons and valves.

The higher the octane rating of a gasoline, the slower it burns and the less likely that pinging will develop. Knocking can usually be eliminated by using the next higher, and costlier, grade - 89, 91, or 93. An occasional knock on a hot summer's day, or when a car is fully loaded or chugging up a steep hill, is OK. Continued knocking should get professional diagnosis immediately.

Tip: If a car runs smoothly, then buying a higher grade of gasoline is just a waste of money. Exception: Some expensive new cars, with a "knock sensor" as part of the ignition system, can increase engine performance slightly with higher octane fuel.

If a car takes a long time to start, or it backfires, coughs, or hesitates when it's running, then try a different gasoline. If using the next higher grade of fuel doesn't work, then switch brands, but stay with the same grade.

When lead was made illegal, oil companies tried a variety of more expensive octane boosters. One of the more popular is ethanol, sold as "gasohol." In concentrations above 10 percent ethanol, or above 5 percent for methanol (another alcohol), the car may be hard to start, or it may stall or lose power.

About two-thirds of the states require that the alcohol content be posted on the gasoline pump, but some manufacturers may stretch the content a bit. If these are problems, switch brands or swear off alcohol entirely.

Some of the new fuels tend to clog fuel injectors, causing a rough idle, hesitation on acceleration, or an engine miss. Cars that are used for short trips or driven in high temperatures appear particularly vulnerable.

That's why the more expensive grades of fuel are being advertised as "high detergent" gasolines. Try a couple of tanks if there are problems; otherwise, stay with the recommended octane rating.