so up here in canada i'm running 87 octane through my '89f-150 5 speed and getting 12.5 mpg. a licensed mechanic told me to try running 89 octane for one tank and see how it performs. same driving style. What should happen if anything? just something to ponder. thanks guys.

 

It won't do much, if anything, unless the vehicle has been tuned for higher octane (ie, timing advanced).