It would be great to have more data available. Specifically, I'd be interested in a view of speed and MPG over time for each trip (either within the app or online. these views are similar to the views provided by Nike+ Fuelband).
I would like this too. I know the mpg is not static at any speed so such a graph would need to be taken with a grain of salt. but it would give a good picture if I can look at the average mpg during a certain period of time. when I can set the cruise control on a level and straight grade of highway and experiment. maybe it can detect. if you have been moving at a constant speed for 1 minute or so, and the road is pretty level, then take the average mpg for that speed. then we can go 5 mph faster and do that for a few minutes. and so forth.