Would be great to be able to record average MPH at each fill up The ability to plot MPG on a graph alongside average MPH would also be great. City versus highway mileage is basically a total guess for me. Most cars now have the ability to track average MPH.
You could definitely use the Notes field to record this info right now. That would make it part of an export and you could use something like Excel to graph that.
I'm not sure average MPH is as common as you think. This is our first request to track it. We're pretty strict about keeping the fuel-up form extremely simple so we're not likely to add it as a separate metric to track.