Originally Posted by
petrified.rabbit
i'll post in the zombie thread.
it is likely the switch octanes of the crap you/we all call "gas" will increase mpg. But it is not the Octane change that increases fuel mileage. It is most like the amount of ethanol in the gas. You see those pumps that say up to 10%? well regular most likely has the 10%, where as premium might only have 8% Super might have 6% .(regional names? 87, 89, 91ish respectively). ethanol is not as fuel efficient as gas is. So if you run 100% true fuel, then the mpg wont change with octane changes. but the displacement of the ethanol will..
How ever at the same time you risk having unburned fuel into the exhaust, eventually clogging your cat and seizing the engine. You might pick up timing issues. The added octane does not burn the ethanol more efficiently.. its just there possibly is less there.
If you find a gas station that uses the same amount of ethanol in every grade of fuel. you will see the same fuel mileage.
The best thing to do would be find a filling station that does not add ethanol, and buy their 87 octane. It would probably provide the BEST fuel economy, but good luck with that i think when gas prices went over $3 a gallon, ethanol got put into everything.