If the air temp is high that can give 10% more mpg (or more). If you drive 60 that helps a lot also (vs going faster). Tailwind can make a big difference. Technically, higher octane does not yield more mpg, even though "everyone" says it does since it is "common knowledge". But octane has nothing to do with energy content, only resistance to preignition.

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra