Adding a partial to the next fill-up.

[markj]

3snakes is right, there is no reason not to calculate the MPG on a fill-up after a partial fill up. Simply add up all the fuel added since the last complete fill-up and divide by all the miles since the same fill-up. You just have to realize that the MPG value for a fill-up is not the MPG since the last (partial) fill up, but since the last complete fill-up instead.

Like DTMAce I used to use a spreadsheet to track this stuff BF (before Fuelly) and that's how I calculated my MPG column. The thing is when you plot a graph of the MPG over time, you have to treat the partial fill-ups as missing data (not zero) and omit them from the graph.