It's hot, VERY hot today here in San Jose, so this afternoon instead of roasting at home I decided to do some serious driving
I started out driving an easily repeatable roughly 1 hour long drive, cruising at 70MPH, with the last of a tank of 91 octane gas.
Then with the very last bit of that tank I weighed the truck (4980lbs with me) and then did three 0-60 runs with my GTech Pro Comp.
Then, with the empty light on and DTE at 25, I filled up the tank with 87 octane (17 gallons worth). Repeat the trip to the scales (5060lbs) and test 0-60 runs with the GTech, and lastly drive the same hour long test loop cruising at 70mph.
The results? HP wise, it's identical, measuring 194HP at the wheels and 0-60 times of roughly 7.4 seconds (yes that HP number is low, my GTech always reads low, that's okay, the before and after comparison data is still valid and it was virtually identical).
The MPG? 91 octane clocked 22.7MPG while the 87 only got 21.6MPG. If you do the math, and assume a $.20 difference between 87 and 91, gas would have to be over $5.00 a gallon for there to be any economic incentive to running 91.
After putting in the 87 I did occasionally hear some brief pinging when in transient partial-throttle situations, but after a few minutes the ECU seemed to have compensated properly and I didn't hear any more for the rest of the evening's testing. If the detonation comes back and is at all persistent I'll move up a grade to 89, but there really doesn't seem to be much reason to run 91 beyond it giving you that warm fuzzy feeling inside from spending more money to burn slighly less gas (one gallon less per tank roughly).
Next time I have a toasty afternoon to kill maybe I'll try 100 octane for giggles and see if the timing maps have any headroom to take advantage of it