I do not, and it might be more ambitious than you want to be, but there are devices, such as
https://yellr.com/index.htm , that allege to make it easy.
Or just buy a GPS and use that to measure your actual speed. My primary vehicle over-reads by a couple of MPH at 60. I figure this is actually mostly good, keeps me from speeding quite as much as I think I am. Yes, it's bad in terms of warranty coverage, but I don't put a lot of miles on--my warranties run out from time, not mileage.