No, it's not a silly idea. I think quite a few people would like it. When we get feature requests like these we ask ourselves a few questions to see if it's a good fit for the site. Right now our mantra is "keep it simple". So we don't want to add a field to the fuel-up form that only a small group of people will use. We have to look at the big picture.
For now you could export your data and total up the trip computer data in Excel. That would give you a sense of the difference in overall average.