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My Bridgestone Battle Wings are almost worn down to the wear marks - I'll be replacing them over the winter. But, the other day when I was doing some highway riding, I happened to bother to plop my GPS on the bike. I set my highway cruise speed and looked at my bike's speedo - it read "80 mph"....I thought "people must be flying today - they're just wizzing by me".....

I then looked at my GPS readout and it said I was barely going 70 mph! Combine the worn out rear tire with the Strom's speedo error, and you're really trolling in the slow zone!

I had no idea I was so far off.....
 

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Vee or Wee?

I thought hat the Vee speedo was off the engine so it wouldn't be affected by worn tyres.
 

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Vee or Wee?

I thought hat the Vee speedo was off the engine so it wouldn't be affected by worn tyres.
What would happen to that value if you installed a different size rear sprocket? For that matter, what if I took the wheels completely off your bike?

See the problem with your assumption? :mrgreen:

The speed of the vehicle is distance traveled travelled over time. Our bikes' speedometers assume a particular relationship of this value to some other value that can be measured easily on the vehicle -- either the number of rotations of the wheel hub over time (Wee), or the number of revolutions of some other part (Vee; presumably the engine output shaft) over time.

Either way, the resulting distance traveled depends very much on, among other things, the wheel and tire diameter.

The only way to accurately measure an object's speed is to measure its distance traveled relative to some fixed point in its environment (not something that moves with the vehicle), over time.

Any method that measures the movement of something on the vehicle relative to something else on the vehicle, and then reports vehicle speed, will be making some assumptions and guesses. The Wee's speedometer, in fact, is subject to fewer assumptions than is the Vee's, because there are fewer relationships that can change between the hub and the contact patch than there are between the output shaft and the contact patch. Both measurements, however, will be increasingly optimistic as tire diameter shrinks.

GPS-based speed measurements, by the way, have their own set of assumptions, which someone more qualified than me might explain. One that comes to mind is that basic GPS will not account for changes in altitude; it will underestimate your speed on grades, to some extent, because it only knows how to measure horizontal position changes (it measures your position relative to the leg of a right triangle, whereas you're travelling along the hypotenuse). Maybe there are some devices that do account for this.
 
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