With recent trips to Montreal and MINIsOnTop, I have seen MPG averaging close to 30 – GBMINI loves long runs at sensible highway speeds:
GBMINI#3 MPG chart
Conversely I get my worst MPG in cold weather when the engine takes ages to warm up and I drive just the short (11 miles) distance to work.
I also expect MPG to drop 10% if the JCW upgrade gets installed (if it is ever released/approved for the auto transmission!)
Overall, my good MPGs on GBMINI#3 are within 5% of the best on GBMINI#2, before that car got the JCW upgrade.
The auto transmission clearly does not hurt fuel economy very much – driving style has a much worse effect!
M&I / GBMINI 
Looks like you’re using Excel? How are you dealing with missed values?
I’m using the following as inputs to my own spreadsheet:
* date
* odometer
* gallons
* cost
* OBC’s version of MPG
* actual MPG
* OBC’s average speed
I then calculate the actual MPG, difference between actual and OBC values, miles/day and $/gal.
When my MINI was in the shop (again), they filled it up for me, and I don’t know how much they put in. I’m sure this will happen again. I just don’t know how to deal with it as far as the formulas go. Are you doing anything to prevent “#VALUE!” or “#DIV/0!” from showing up in the computed fields?
Grrr. For the record, comment preview looks nothing like the comments once they’re posted.
You’re right about the preview … I never use it. Maybe I should just remove it as an option!
I don’t have any missed values. But if I did, I think I would “best guess” to fill them in.
My Excel has:
Date, Mileage, Gallons, Cost (info from the fill-up)
Distance, MPG, $/Gallon (calculated)
I don’t record the OBC stuff – on GBMINI#2 it was too way out, and on GBMINI#3 it is close enough to be useful!