Shotgun, what MIGHT be instructive would be to add a "Cumulative MPG" column (total miles/total gallons) to see if it came any closer to the digital readings. It would also tend to eliminate variances due to wind, hills, driving speeds, etc.
To be really useful, I think I'd try to separate the towing periods from the non-towing periods (which you could do with a simple boolean switch - key off a "Y" in a "towing" column or something).
You have the data, so it would be an easy mod and enhancement to your current display - I'd have to start from scratch, since I always do mine by hand on the back of a gas receipt.
- Jack