Recovering faster from partial fill-up

[NBtarheel33]

This is precisely how I deal with the "partial fill-up" problem in my own spreadsheet, and even go one step further, to avoid having blank entries:

You know that you went 210 miles on 12 gallons; that's 17.5 MPG. So, I go back and fill in the MPG for the 3-gallon "tank" with 17.5, and the 8-gallon "tank" with 17.5.

Now, I also like to track how different brands/octanes do over time. Obviously a partial fill-up messes things up here. But now I can go back and run the calculation, including 3 gallons of brand A at 17.5 mpg, and 8 gallons of brand B at 17.5 mpg. Obviously, it's not perfectly accurate, but it's close enough to get an idea of what's going on, and considering that I've only had three partial fill-ups in the last three years (precisely to avoid throwing my calculations off!), things smooth out nicely over time.