My car does not have a reliable gas gauge, but I know I need to fill up after 250-275 miles. If the previous fillup's odometer reading were displayed somewhere on the new entry form, it would greatly help me.
I'm using fuelly with my iPhone for the most part.
Still, it seems pretty doable to have the example miles that already show under the input box relate to the last fuelup. If it's the first fillup and there's no previous data, use the random example numbers that already show.