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speedo speedometer gear sources

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scott View Drop Down
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Joined: Jul/10/2007
Location: Wildwood Pa.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote scott Quote  Post ReplyReply Direct Link To This Post Posted: Aug/16/2019 at 3:28pm
I prefer to use the speedo rather than the odometer to adjust the driven gear. It works the same as how tom calculated it. Drive at the speed you want the speedo to be most accurate, say 65 as indicated on speedo. GPS indicates 58. 65/58=1.12 or 12% error. If you have a 18 tooth gear, you need to increase tooth count by 12% or 20.1 tooth gear.

My reasoning for using the speedo rather than the odo is this. The odo is directly drive off the cable, & will have the same percentage of error at all speeds. The speedo is not directly driven off the cable, it is magnetically driven, & the percentage of error can vary by speed. At 25 you may have a 2% error, 50 a 5% error, 80 an 8% error.  So, you pick the MPH indicated that you want it to be most accurate & use that for the calculation.

In reality, either method will get you as close as possible. Often the calculated "correct" gear will be something like 18.65 so you use a 19 tooth. I've been lucky & have always come within a couple of MPH at 65.
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