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Speedo Accuracy / Tyre Diameter


Well Known and Active Website User
VOC Member
Last night I rode over to Rick Schunk’s house to do some wheel/tyre research. We rolled his B Rap with 20” front for three front tyre revolutions and measured the distance, then did the same for my C Rap with 21” front. The 20” circumference was 78.5” and 25” diameter. The 21” was 83.5” and 26.6”. So, theoretically my speedo should read 94% of Rick’s. Now, the question is, how accurate is the standard tyre, various gearsets, and chronometric combo? A GPS could be used to check, but I am too thrifty for that. Maybe I’ll try calibrating my car speedo while in cruise control to milemarkers on the highway using a stopwatch and then getting one of my fine young sons to drive in cruise control at a true 60 mph,
while I pace him and observe what the Vin speedo is indicating. What will I do with this info? At best, know how much I really need to exaggerate how fast I was going while swapping tales of speed and daring.

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