Speedometer Gears - 74 CJ5 (and others)

Got the CJ on the road after many months of rebuilding. Speedometer doesn't work. Speedometer and cable are new so I checked out the gears in the transfer case. Driven gear looked fine so I used my iPhone like a periscope to check out the driven gear and shot a video as I turned the drive shaft. Looks like the gears are chewed up in a few spots. The PO had told me the speedometer was way off and he had no idea what the real mileage was. He had changed from stock tires to 33". So, since I am going to be replacing the gear anyway I might as well try to get the speedometer working accurately. Can someone here please verify my logic on this: The speedometer performs two functions, speed and distance. I am going to work this from the distance perspective because it seems simpler to me. If my odometer reads out one mile for a mile driven then the speedometer should be accurate too. The 74 CJ5 came stock with F78-15 tires which should have a diameter of 24.4 inches. Pi * diameter gives me a circumference of 86" or 7.17 feet. This tire should rotate 736 times in a mile. I have 4.27 gears so my drive shaft should rotate 3143 times in a mile. My speedo drive gear is the yellow 7 tooth and the driven gear is 20 tooth. So my speedo cable should rotate 1100 times in a mile in the stock configuration. So I need to come up with a speedometer gear ratio that provides the same rotations in a mile with 33 inch tires.

My 33 inch tires actually measure to 32.75 inches. Pi * dia gives 103 inch circumference or 8.6 feet. They will rotate 615.8 times in a mile and with 4.27 gears my drive shaft will rotate 2629.5 times in a mile. The 7/20 speedo gears will give the speedo cable 920 revolutions in a mile for an error of 16.3%.

I have three sets of driven gears on hand to work with - 18, 20, and 22 tooth. The drive gears are available in 6 tooth (green), 7 tooth (yellow) and 8 tooth (brown). I found a replacement for the yellow gear and the 8 tooth brown one. So now all I need to do is calculate the speedo cable rotations per mile for all the available gear configurations and see which will give me 1100 revolutions in a mile. My options are 7/18, 7/20, 7/22, 8/18, 8/20, 8/22. With a 6 tooth drive gear none of the ratios work out. The two that come closest is 8/20 which give me 1052 speedo cable rotations per mile and 8/18 with gives me 1169 rotations per mile. The first gives me an error of 4.4% and the second gives me an error of 6.2%. With the first when I am going 60 mph my speedo will read 57. With the second when I am going 60 my speedo will read 64 mph. Since this will be my sons jeep, his first vehicle, I think I am going with the second option; 8 tooth drive gear and 18 tooth driven gear.

If someone smarter than me could check my math on this I would greatly appreciate it.

 

I'm not going to argue with your math. You are a lucky guy to have all of those gears, some people have been looking for some of those for years. Your other option is to go to a GPS speedometer.

 

A few things to consider. 1) My data shows a F78-15 tire is 27.2" dia. not 24.4" dia. Might want to check your data. 2) When I was doing my speedometer I found information indicating that for a speedometer to read correctly, it requires an input of 1000 revolutions per mile at 60 mph. 3) Going back to tire diameter. I found that the tire diameter data the manufactures publish is for a tire with no load on it. When a tire is mounted and the weight of the vehicle is on it, the effective diameter is less the published diameter which skews your math. I know kind of splitting hairs but it does effect the accuracy of your math / speedometer.

 

Not true. The distance per rotation of a tire only depends on its circumference. This does not change with the tire on the ground or not. The only way the distance traveled can change is if the tread stretches when the tire is inflated.

 

Thanks. I'll rerun the numbers with 27.2 diameter for the F78-15.

 

http://www.quadratec.com/jeep_knowledgebase/article-46.htm

This page should help. Rugged Ridge caries an assortment of speedo gears if need a different one.

 

Those are not the gears that go in my 74 Dana T 20. Also for some reason they don't list 4.27 gears which were a factory option in 74. I'll try to post a picture of the gears I a using.