I got 40mpg once on my way from Tucson to Boulder. I must have picked up some super gas in Hatch or something, cuz it was back in the 30's again on the next leg. I think maybe the air was really thin, so my ECU adjusted to just dripping gas into the cylinders

Anyway, you didn't mention if you put any injector cleaner in your latest tank. Did you try that? If you didn't, do it. If you did, that would explain the jump to 25 and then back down to 20.