First a bit off the subject:
I have a highly modified 06 Pontiac Solstice, 2.4 naturally aspirated, 143 rwhp and 135rwtq at the wheels. 26-28 on the highway and 23-24 around town. After spending 1/2 the price of the car on modifications (MP62 supercharger, intercoler, 3 belt pulley assy, Exedy stage 2 clutch, header, high flow cat, custom exhaust, custom body panels, custom leather interior) I now have 270 rwhp, 280 rwtq and get 29-31 highway, 23-25 around town. No I will never get my money back, but you can't wipe the smile of a my face. Car is 3 seconds quicker 0 to 60 and all most 4 seconds in the quarter. Worth every penny.
Now back on topic:
01 crew SC (same one MP62) only mod is a K&N CAI. I have had the truck 8 years. In that time it has always drank 93 octane. I see 15.5 to 16.5 in combination or highway did not matter. Over the last month or so I have been experimenting. Removed roof rack .1 to .2 gain to the gallon over several tanks. Then I did the unthinkable, I put 89 octane in it. So far the first tank was 17.2. Second tank was 18.7 but that included a 60 mile trip without a lot of stop and go (saw 20 for about a mile). I am on my 3rd tank of 89, all mixed driving and I am sitting at 17.4 (added a soft cover on the bed) So by dropping to a lower octane that is up to 10 cents a gallon less I have improved my gas milage by 9-10%. Will have to continue and see if I get any knock or if this heat causes and issue, but so far it hasn't