What I've seen from the members on GreenHybrid so far, they seem to be willing to make an effort when it comes to mileage...
But I can see your point, and I guess you know your members a lot better than I do. How about recommending to find out the average deviation on the display, and then recalculate before entering the data here? In my case I would take my display mileage of 48 and take away the average 10 % to get 43.