For those of us without a GPS, ..... if the road has mile markers, then 60 MPH is 1 mile per minute and one can then calibrate as Mike suggests in his post.

For the KPH crowd:  100 KPH is 36 seconds per kilometre (100k/hour = 100k/60 minutes = 100k/3600 seconds, divide the numerator & denominator by 100 and you get: 1k/36 seconds) m2cw   m

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From: Mike S <mikes@FLATSURFACE.COM>

I've "calibrated" by using a GPS to determine my actual speed. I drove
so the GPS indicated 60 MPH, and noted what the speedo read.