I have to agree with brandiniman on this one. Its pretty obvious an automatic setup couldn't be done but one where the user logs the information and the sets up the next interval on their own would be worthwhile.
I have mentioned this site to a few friends and they have all asked if they could do this very thing. It would certainly help to set the site apart from the other websites that track fuel mileage.
I just want to be another voice that would like to see this feature added to Fuelly. Fuel efficiency is nice, but total cost of ownership seems a bit more important in the long run. Tracking this added piece of data would also be helpful so we could see how oil changes and tune ups affect our mileage.
What I would really like to see is some type of call out on my recent fuel-ups graph that shows when I've done maintenance to my car. I'm imagining something similar to how Google Finance shows news on a stock's market graph.
The advantage of this would be to easily explain why we might see a mileage bump in our driving.