Haha! Yeah, knowing where the town of Index is, (having spent some time nearby in Gold Bar and having been to Index a few times), leaving there is usually pretty much all downhill using much less fuel and coming back is going uphill using some fuel. Unless you are going UP the mountain from there in which case it's mostly uphill leaving and downhill on the way back. 22 mpg seems a bit high to me but I don't know how your specific truck is set up. If you are actually getting 18+ mpg on a flat highway like an interstate hwy, running 65 or maybe 70 mph I'd say that's pretty good.
I suspect that the smaller size winter tires were tires that you already had on hand so you put them on the truck. I'd wait until you have the correct size tire before trying to calculate mpg, and that is mainly related to engine RPM at various speeds.