Cant understand my milege

[RSlater]

On a recent trip to Reno and back, I drove 65-80 the whole way (route via Jordan Valley is pathetically enforced @ 55 mph, but I can't go that slow) and got 17. 3 hand calc'd on the way down vs 21. 9 overhead. Ironically, I drove a bit faster on the way home (75-82) and got 17. 9 when returning to Boise while the overhead showed 23. 0. My truck has always been a horrible liar (although I attribute part of that to my Smarty), and the overhead is typically about 21% higher than reality or about 5 mpg during my normal commute. Since I live in the mountains, there's no such thing as a "flat" commute for me, but my most recent fill-up gave 19. 6 mpg overall (80% highway @ 60 mph). Much better than the 15-16 I was getting!