it wont necessarily run rich but hes just adding more octane which would be useless if he doesnt tune for it. with 91 octane he could run more timing but the fact that its still on factory tune it will only run the amount of timing that the factory tuned for.all your doing is making your car run rich by putting a higher octane rating that it doesnt need in it. factory tuned for 87? use 87
if you get it tuned for 91 octane, THEN it will perform better on 91
thats somewhat inaccurate as you do not drive EXACTLY the same everyday.I did a MPG and "feeling" test. If I go down to regular pump gas (87) I noticce about a 20-25% drop in MPG and a definite performance drop.
Same gas stationthats somewhat inaccurate as you do not drive EXACTLY the same everyday.
Also, there would be no increase in performance due to your car not being tuned to the higher octance.
Even if there was a performance increase it would not be significant enough to notice the power gains.