Are you wasting money and increasing your running costs by buying more gas than you need? Check your owner's manual for the octane number your car requires to run efficiently. The number you are looking for is called a RON rating. Usually regular is rated at 87, while premium carries a rating of 91. The RON number is a measure of the gasoline's resistance to self-ignite. When it does self-ignite we refer to that as pre-ignition (the fuel has exploded before the spark plug has fired). That's bad. The driver would detect pre-ignition by a rattling noise in the engine, usually under acceleration. Extended driving with this condition will damage the engine. So check the manual and don't buy more octane than you need.

Q. According to my manual my car requires premium gas. In error the attendant filled the tank with regular. I have had all kind of comments as to the damage this will cause. Some of my colleagues even suggested that I would damage my converter. I don't know what a converter does, however; I do know they are expensive. My question is: What damage will putting the wrong fuel in cause?

A. It always amazes me the rumours and urban legends that are out there. No damage will result. In fact the only thing that you may notice is a reduction in performance. Today's cars are very reactive to their environment, that is they will adjust to the prevailing conditions. In your case the knock sensor would pick up any detonation in the engine caused by a lower grade of fuel. The computer will take the knock sensor readings along with the information supplied by the other sensors and reset the engine. No damage will result, however; you may also experience lower fuel mileage as the engine is not running at peak efficiency.