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Originally Posted by KBerryhill
We just filled up our HiHy for the third time. We decided to try 89 octane, since we calculated that it would only need to improve mileage by 1.5 mpg to be cost effective. On the first hour of the new tank, mpg is up 2.5!
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Ok, I don't know what the highlander is rated for BUT using HIGHER octane gas then what a car is rater for will NOT i repeat NOT help your gas milage and NOT I repeat NOT give you more horse power. Higher Octane is used for engines that require higher compression, this way they can be designed to run at higher compression generating more horsepower but it will NOT save gas. Higher octance gas actually has less energy in it (the octane rating specifies at what compression it will explode and not energy level!).
So don't use higher octane gas, it will not help you, it will pollute more and cost you more money.