Jason,
A couple of things. When I added my first tank info (37.8 MPG) the average showed up as 46. Now that I have added all of my info, you show 46.8 average while my excel spreadsheet shows 46.2. Maybe it's the weighted average thing you mentioned.

Also, under driving time, I assume that you mean average trip time. You can only choose up to 240 minutes. I suspect you need to go up to 720.

It would also be good info to have a place for average speed, tire pressure, AC on/off and canned answers for driving strategy. These are all things that drive my MPG. A cumulative mileage would also be nice, displayed on the summary screen.

It's a Wonderful Day for Science!