The kind of car you drive may help determine which kind of gasoline you should put in the tank. The octane rating that's best for your car's engine should be listed in the owner's manual. Octane ratings for gasoline are calculated according to its ability to reduce knock, or pre-ignition of fuel in the engine. Engine knock can be most prominent when you're speeding up or pulling a heavy load. Having an improperly tuned engine can also sometimes cause knocking. As your car gets older, it may be necessary to use a higher octane gasoline. If your car knocks severely and often, it most likely isn't an octane problem. A qualified mechanic should check loud and persistent knocking. It's usually a good idea to buy your gas from a reputable station that does a fair amount of business. When gasoline sits in a storage tank for too long, water or other impurities may collect, and eventually end up in your tank. Consult your owner's manual or a qualified mechanic if you have further questions about which type of gasoline is best for your car.