I was thinking the same thing.I use the Audi A4 race car for the Extreme DTM Nordschliefe race - 83,000 per lap.
I also call this playing the game, not grinding.
I haven't had to do too much of it so far, but I made a conscious decision to use a less expensive production car as much as possible -as opposed to a race car or uber-exotic supercar- for any XP/money farming, for the sole reason that it would be much cheaper to do a an engine/chassis refresh considering the mileage such a car would accumulate..
I was also worried about the accumulated mileage but instead of less expensive production cars I have been using my Gamestop pre-order cars (McLaren F1 Stealth and Joey Lagano NASCAR) because it only costs $500 to perform an engine/chassis refresh.