Because I have seen mods with working odometers that got me thinking. Can we have default cars with working odometers in them? The CB-40 has them, but you may not have noticed.
I think i remember someone bringing up a bit of mileage code in the game files, that had calculations for reducing engine power and efficiency, likely for career mode. i'd imagine implementing an odometer somewhat like the gear indicator or the ETK 800's text display would not be that difficult.
I think its also fully possible to make the odometers fully work on cars with analog odometers. the CB-40 mod has working analog odometers.