I've been wondering about this gas question too. A couple tanks ago I put in 87 Octane and measured about 22 mpg. I thought that was low. By accident, I put 89 octane in with my next tank and I got 25 mpg. Could the knock sensor be effecting the timing that much with the 87 octane? I still think 25 is low for mixed city/highway commuting. Maybe less ethanol in the last tank..?
I have an old Mustang 5.0 and it gets about 20 mpg with a V8