Medical Dave,
I'm just as ana... ummm, German as you and I track every tank in an Excel spreadsheet, which computes the average (per year) as well. Handy stuff to have around so you can tell if something caused a sharp drop in MPG for whatever reason.
On a side note, since you live in a cold area, why don't you use synthetic oil? It makes a HUGE difference in cold starting ability and also lubes your engine during the first couple minutes of operation, where the 15W-50 dino is basically molasses in the sump. You can extend the drain intervals to offest the cost. I use Delvac-1 and change at 10k (have oil analysis to back that up). Rotella 5W-40 Synthetic is ~$13/gallon at Wal-Mart, it's not the best stuff but would be better than the dino Rotella...
Regards,