@Sean Seena posted:(Double ps, for posterity & completeness' sake, one way to measure speed error is have a modern car with an accurate speedo go X mph, sync your speed with them, read your own gauge. This is what I did while driving the car home on the first drive. But a smarter way is to simply download one of many free speedo smart phone apps. Or, of course, a GPS device if you have one.)
Or, you could do it the old fashioned way: with a tachymeter.
Head down your favorite road, spot a mile marker, when it crosses the plane of the windshield support, hit the button, hold your speed steady then hit it again at the next mile marker. Checking at 60 mph (100Kph) is the easiest, because it's a "mile a minute." A tachymeter face has a scale to confirm your measurement at other speeds. 
I'm guessing that my speedo has been recalibrated, because although I don't know how it was reading with the 165/80's on it, after I cleaned the 30 year old grease out if it and put a new needle on it, it reads 100Kph at 59mph with the 185/65's.