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Is there any difference in MPG when you use 91 vs 87 gas?

5500 Views 9 Replies 7 Participants Last post by  ceric
Is there any difference in MPG when you use 91 vs 87 gas?

I'm currently only using the 87 octane at the gas station for obvious reasons. The gas price is here in the SF Bay area around $2.51 although I got gas last week for $2.41 at Costo. I get roughly 52 MPG with freeway miles. Not bad.

Has anyone checked out to see if there's a difference between the 87 and 91 octane or perhaps the 89.

Thanks for replying. Happy Prius driving :D
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Unless you have the VERY unusual situation where you have knocking robbing you of power, NO!. Actually, you will lose some MPG, as higher octane has LESS energy in it for the additives that increase the octane.

Octane rating is the resistance to self detonate at high compression, not the content of energy.
Yes, some engines do require higher octane because they have higher compression ratios on an Otto cycle engine. Because the compression is so high, regular 87 octane would pre-ignite and cause pinging, power loss due to igniting too soon, and probably eventual engine damage. You want the spark plug to determine the time to ignite, not subtle random events that can't be controlled.

Hi compression yields more power mostly because of the longer power stroke in the 4 cycle engine. Since a longer stroke means more air/fuel mixture being shrunk to a smaller space, you have higher compression.
Some alternative engine systems, such as our Atkinson (some think it more of a miller) reduce the compression while still having a longer power stroke by reducing the intake stroke, or releasing some of the mixture taken in during the intake stroke during the early part of the compression stroke.
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