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 c
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Fairfax County General