Doing burnouts the first time driving a car could lead to very badly calibrated odometers. This should reduce that problem, although it's still a good idea to set off with a minimum of wheelspin up to 50km/h the first time you drive a car to help it be the most accurate.
If the app has a car calibrated wrong, navigate to the app's folder (steamapps\common\assettocorsa\apps\python\stereo_odometer), open odometers.txt, and in the [Adjustments] section, delete any lines that are wrong...
Apparently the speed I was reading from the API is off by some mystery factor, times the tire radius (for example with a radius of 30.5cm, the factor is 3.45 - I'd have expected it to be 2pi or something, but it's not)
The app can work out this factor once the car's hit 50 fake-units per hour, so around 55-80 km/h (depending on tire size). Once this happens it'll adjust the display to show accurate distances and store the factor for future km logged on that car.
Stopped it from recording mileage during replays.
Squished up the border a bit. If it's not narrow enough for your tastes, go to config.ini and change the squish parameter. 0.7 is the maximum I'd recommend, 0.4 is the default (was 0)