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10.00 x 22 Tire Diameter

Posted By Ol' Kenny Sunday, July 07, 2019 12:08 PM
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Ol' Kenny
 Posted Sunday, July 07, 2019 12:08 PM
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Last Active: Monday, September 30, 2019 1:24 PM
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Hi Guys,

I switched wheels and tires on my W900A from the old tube type 10.00 x 22 to 11R x 24.5 tires and now the speedo doesn't read correctly. I found a speedo shop that can re-gear it but I need to know the diameter of a 10.00 x 22 tire. There was a really good article in the last Wheels of Time on road speed with a diameter chart but 10.00 x 22 was not listed. 11R x 24.5 is listed as 43.3" diameter.....10.00 x 22 should be close to 42" diameter but I wanted to verify with some experts. Thanks for the help!
Geoff Weeks
 Posted Monday, July 08, 2019 1:08 PM
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You don't need the old tire diameter. You need the RPM (revs per mile) of the tire you are going to use, the rear axle ratio and the tooth count of the gears in the trans for the speedo.
Trying to adjust what you have now from a guess of what you had is a recipe for failure.
A 10X22 and a 11x 24,5 should have the exact same RPM for comparable tread design.
If your old tires were 10x 20 they might be close to 42' but rpm is the best way to set a speedo. The tire mfg data sheet should give the RPM for the tires you have now.
Ol' Kenny
 Posted Tuesday, July 09, 2019 1:05 AM
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Thanks for the info Geoff, I am glad I checked before going on a wild goose chase. I did some research and 11R x 24.5 tires are rated at 476 revs/mile. The rear end has a 5.29:1 ratio. Any idea on an easy way to get the tooth count of the gears in the transmission? The aux. transmission is an 8345C Spicer. I removed the speedometer ratio adapter from the transmission and the current one has a ratio of 1.5867:1 and reads too slow. The input shaft to the ratio adapter has 14 teeth.
Geoff Weeks
 Posted Friday, July 12, 2019 4:54 AM
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If you don't want to remove the tailshaft bearing cover and count the teeth, you can turn the output shaft and count the turns of the speedo "pencil gear". You can do it with one turn of the shaft, but it would be better to do 10 turns to get a better idea.
Lets say for 10 turns of the output shaft the pencil gear turns 3 turns. you now know the ratio of the speedo drive and can get an adaptor from there.
There is another option, which may be worth doing if you have a really long cable run. Dakota Digital makes a "box" that converts an electronic signal into a cable drive to drive an old speedo. You can then either use an "adaptor" and the trans end to attach to the pencil gear and give a pulsed output or you can replace the speedo gear on the output shaft with a "toothed gear" and drill and tap 3/4x16 tpi for a mag pick-up. Once you convert to electronic drive, speed correction can be a simple as pushing a button or dip switch selection. It will not be cheap, but if you are contemplating future tire or ratio changes, it would be more cost effective. http://forums.aths.org/Uploads/Images/1244becc-aacb-41dd-b4cb-1b7a.jpg
 prd_886.jpg (83 views, 16.01 KB)

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