My 4 cyl got more economy (25 mpg average) with the lower octane stuff AND had more power. I drove to Ouray, CO in '02 with 3 adults, a canopy, tool box, spare parts and 3 spare tires (ended up needing all of them after my alignment was wrecked by a delivery truck in CO)
Total weight was 6,000 lbs. 2.5L with almost 200,000 miles, ax-5, NP207, 4.10 gears and 30" tires. We cruised at 70 mph most of the way (I had to downshift only when I tried premium gas in the mountains)
The reason for this is that modern higher octane fuels have additives that slow the burning rate and in so doing cut the power drastically but the motor doesn't ping.