I noticed the same thing, we go fishing on the E. Carson river quite a bit and our first trip from the Central Valley over Carson Pass we got 18.1 mpg, put a few more miles on it and the next trip was19.4 mpg. The 21.7 we got on our last trip over the pass was with 1654 miles on it. I just turned 2400 miles and am looking forward to seeing what i get next week on our next trip over there.
I just saw you are in Ceres. I swear I get better mileage on 99 than I do on 5. I’m farther north in Lodi and travel both frequently. Ironically that crazy mpg I got following a school bus was on a field trip down by you out in hilmar.