I realise that it's very much a 1st world problem, but being a bit of a stats nerd I find it quite irritating - I always try to add the "Since Refuelling" mpg reading into my SpritMonitor entries, and every now and then I run a comparison between the actual MPG and car measured MPG (I think there's generally a 5% difference between them).
Long term resets every 100 hours I think, and Since Refuelling does (or should do) what it says on the tin. Since Start resets when the ignition has been off for 2 hours, but these days they are all generally resetting every time I start the car - I was very surprised last night to see that the Since Start reading hadn't reset after being in a Heathrow carpark for 90 minutes (unlike my bank balance when I paid for parking!)