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Type of gas

Does it really matter what grade of gas you use.

This is a topic that has been beaten to death in this forum. Just scroll back through the past few hundred messages and you will find a cornucopia of opinions on the effects of fuel octane. Unfortunately, no one seems to have ever actually documented any scientifically collected data on mileage and engine life that would support their opinions.

In brief - most cars run just fine on regular. If they were designed for higher octane, they will detect a knock and will automatically retard their timing far enough that the knock goes away. Retarded timing makes the car feel less powerful, and may degrade fuel mileage slightly.

I have an old Volvo that has fixed timing that can’t be adjusted, and it does not adjust its own timing when it knocks. That car requires 89 octane minimum or it rattles like crazy on uphill grades.

My wife’s 2004 BMW will run on any fuel, but it runs noticeably poorer on low octane. It runs well on 91 and better on 93. It seems to get about 2 mpg better mileage on premium, which about pays for the cost difference in the fuel. I have not measured that scientifically, however.

My two daughter’s 1997 BMWs run just fine on any fuel, but they are ‘peppier’ on premium.

Depending on the vehicle, yes, it does. Your owner’s manual will tell you which grade of gasoline is required for your vehicle. Use what the manual recommends. Doing otherwise is asking for trouble.

Repeat after me:

High Octane gas does not help an engine that does not need it. Octane is not a measure of quality or power.

Octane is a measure of how easy the gas starts to burn or how fast. High octane means it is hard to start burning. On the other end diesel is measured by certane and a high certane number means it is easier to start burning.

High octane gas may or may not have more additives like cleaners in it, but with today’s regulations any gas has enough for most engines.

   The owner's manual will say that it must have high octane or that it may use regular but should use high octane.  The parts the leave out are:

  * If it needs high octane, using regular can damage the engine.

  * If it should use high octane then it has a system to sense the low octane and to make adjustments to the car that will protect the engine.  However the adjustments will result in lower mileage (reducing or wiping out any economic advantage) and will reduce the power, defeating the reason most people buy cars that require high octane. 

  * Using high octane in a car that does not recommend it very very seldom causes damage but cost you more and offer no advantage.