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Speedometer error

39K views 142 replies 84 participants last post by  Missing Link 
75mph on my bike and 64mph on wife's car. (No wonder people were passing me like I was sitting still.)

65mph on my bike and 55mph on wife's car.

55mph on my bike and 46mph on wife's car.

45mph on my bike and 38mph on wife's car

30mph on my bike and 26mph on wife's car.

A whopping 14% off.

I could understand a 2 or 3% difference, but 14% is rediculous. I really don't care for doing math while riding, takes the enjoyment out of it.
 
I just crunched the numbers you have posted here. I see as high as 19.6% and as low as 15.4% difference when using the car speed as the reference. If you average these out I get 17.6%. So a couple of things here. Either something is pretty messed up on your bike or the gearing has been changed. If you subtract out the about 8.5% or so that a stone stock bike would show that leaves you with 9.1%. So now take a look at a gearing chart and look for a common sizing that would give about 9% difference from stock. 16/41 is 8.9%. So when using your average number from your data above I would guess your gearing has been changed to 16/41. If you use the highest value from your data above your gearing linesup with 16/42 and if you use the lowest value you are pretty dang close to 16/40

What is your gearing? Have I guessed correctly?

-MS
Bought the bike as a 07 leftover w/0 miles March 30th 2011. I just done a 550 mile maintenance yesterday. She now has 640miles as of this evening. My wife's 08 Saturn Aura XE is off of our Magellan GPS by less than 1mph under, my 07 Chevy Impala is about 1.5mph under, both cars at 75mph.

I believe the stock is 17/40 which is 2.35. I didn't even think to look at the sprocket because I see no reason for the dealer to change the sprockets. We coordinated the speeds I would run and her beside me, I would point down with my left hand as my daughter watched and relayed to my wife, my wife would call out the numbers on her indicator and daughter wrote my wifes speed down, next to the coordinated speed list that I was testing. We used her car because the Magellan was more accurate to her indicator. I was getting 56mpg when riding steady, aggressive 44mpg, which seems like stock sprocket setup.

I will do a sprocket tooth count and re-run the speed tests. I wish I had a radar gun though.

Edit: I went thru a town today that had a permanent radar sign. I purposely ran 30mph, it posted 26 flashing. That is 15% difference. Sprocket counts are 17/40,which is stock.
 
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