atferrari - you could see if you could get your hands on a used speed radar, that would be consistent and precise.
Reloadron - your onboard computer monitoring the tire pressure reminds me of when I rented a car for vacation last year. About 350 miles into a 1000 mile trip, the computer showed a tire with low pressure. I stopped and added air to the tire which it indicated, but the fault kept showing. One would think that it would be obvious which was the offending tire, as it would visually appear low, but this wasn't the case as it was only a few psi low (like 5). I topped off all the tires & the fault cleared. Turns out the tires had been rotated, but the sensors hadn't been reset, so the fault was being reported on the wrong tire. (It was a 2016 Chevy Cruze, btw.)