If the bloody new-fangled digital LCD odometer can remember the overall mileageº then whyTF does it have to reset the trip meter if you disconnect the battery to, e.g., remove the airbag?
º and a related point, I wonder how, internally, it does that? It can't surely be anything battery fed (even a backup battery) 'cos it has to be able to remember the mileage even if it's disconnected for ages. I guess it could use something like 3 EEPROMs, and erase-and-rewrite them every mile, with the 2 not being written this cycle being used to over-ride a corrupted reading. But I bet it doesn't. Time taken to update is more or less irrelevant, even at 120 mph, miles take 30s and tenths (assuming it records tenths and doesn't display them) take 3s.