Over a few months, I designed and implemented an algorithm for counting the distance travelled off-road. It took a TomTom road map of the whole country (not a cheap license fee), divided the trip into pairs of points, checked whether each segment intersected with or came within 10m of a road, and finally made a total of the distance. There were deviations (odometer compared to on road + off road straight line distance) but it was good enough to be usable, and we shipped it. It also generated reports for reclaiming road tax for off-road driving.

In practice, the client realised that our $7 per vehicle per month fee for the service will only save them $2 or $3 in road taxes for their most off-road vehicle. Just because it's technically exciting doesn't make it financially logical.