Posted by Vaughn [172.69.140.182] on Wednesday, September 20, 2023 at 23:12:48 :
In Reply to: To complicate things even further posted by Clint Dixon [172.70.127.160] on Wednesday, September 20, 2023 at 15:11:45 :
Ok, using the chart at the link below to compute the number of teeth needed on the TC speedo pinion running 1100x16 XL tires (538 revs per/mile, per Michelin Truck Tire Specifications):
NO CURRENT DRIVEN GEAR
NUMBER OF TEETH ON DRIVE GEAR X AXLE RATIO X TIRE REV/MILE/1000 = DRIVEN GEAR NEEDED
(5x4.89x538)/1000 = ?
(5x4.89=24.45x538=13,154.1)/1000=13.1
This is the same result as the formula I use at the post below. All this shows there is more than one formula to calculated gears needed that ends with the same result. You say radius is from floor to center of hub. From the floor to center of hub is 18-3/4, from center of hub to top of tire crown is 19 inches on my truck, not much difference, however this is unload, loaded is 17.8 inches at 80 pounds, so we loose an inch until the truck is in motion, then that figure will change due to outward centrifugal force applied to the tire.
We are fortunate to have a table in the parts book that gives us gears required for the standard PW tire, & R&P ratio which does not result in 100% accurate reading at the speedo, but it's all we got outside of an adaptor or GPS speedo. We can play with pounds of air in the tire to get greater accuracy. Tires outside the parameters of the parts book there isn't many options except those mentioned above.
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