With the price of gasoline what it is today it is important to know what will give you the best performance in your engine. If there are three grades of fuel on a pump most people will respond in one of two ways. They will either go for the cheapest thinking they are saving money or they will go for the most expensive thinking it has to be better because it costs more. If you choose the wrong fuel in either case it can cost you more and has potential of damaging the vehicle's engine.

The ratings at the pump are called octane. Octane is not the amount of energy in the fuel so increasing the octane will not give your engine more energy if it is not the proper octane. Trying not to get too technical the explanation of octane number of gasoline is the amount of pressure it can withstand without combusting spontaneously. The higher an octane number is the more pressure it can withstand before it ignites from the pressure caused by the engine's compression.

Diesel engines are designed to use compression to ignite the fuel but gasoline engines use a spark plug to start the fuel burn. If the compression starts the gasoline to ignite before is it supposed to start burning it will produce a knocking or clacking sound usually called ping. Pinging can destroy an engine and thus should be avoided. Pings also reduce the efficiency of a gasoline engine.

How do you know which octane level to use? First look in the owner's manual if you have it. They will usually tell you what octane level will work in your vehicle. Start with that level. Fill your tank and record the mileage on the odometer or set your trip odometer. The next time you purchase fuel subtract the odometer reading recorded earlier from the current odometer reading which will result in the miles driven (if you have a trip meter this can be read directly without the math). After the number of miles driven has been determined divide the mileage by the number of gallons (I am in the U.S.A so I use miles and gallons but kilometer driven divided by liters works too). Now you know how many miles you get to a gallon of fuel. After doing this a few times change to the higher octane and see if the mileage increases. If increasing the octane increases the mileage check to see if it is enough to warrant the increase in cost of the higher grade fuel. This is done by dividing the miles per gallon by the cost per gallon of each grade tested which will result in the cost in fuel per mile.

The most important test starts as soon as you are leaving the station on that first test fill-up. As you are leaving the gas station listen to see if your vehicle engine pings, if your vehicle does not ping at normal acceleration, when it is safe to do so, try a very fast acceleration preferable on an incline if possible. If this does not result in engine ping then that fuel grade should be safe to operate in your engine.

It is advisable not to operate your vehicle on a lower octane then the manufacturer suggests. If your vehicle does not have a high performance engine and you do not know which grade will work you can start your test using the lowest grade because most of these engines will work well on 87 octane. If your vehicle does have a high performance engine it would be best to start the test with a medium grade of fuel.