Many, if not most, cars these days have trip computers that record average speed and/or total running time. It can be very interesting to plot average MPG against average speed for each fuel-up. The two are often correlated: higher average speeds often reflect more freeway driving and, therefore, higher MPG. Of course, the correlation isn't perfect because MPG is highly dependent on the leadness of one's foot. Nevertheless, the correlation is often pretty good.
I have taken to recording both average speed and total time into the Notes field on Fuelly. Technically, I only need to record one of the two values since the other can be derived because we already record the Miles since last fuel-up.
It would be nice if Fuelly could natively record average speed (or total time) and include average speed on the MPG graphs. It may even be interesting to compare this against City Driving %, which is often a highly subjective guesstimate.