Speedometer Off After Changing Speedo Gear

benny3577

benny3577

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Iowa
I know this issue has come up on the forum before, but I have tried some of the solutions and thought processes to remedy it and I'm still unsure. I am working on correcting the speedometer. My TJ has 3.07 gears and 32-inch tires. I have verified this ratio by checking the tags and turning the tire and watching the driveshaft spin.

I recently swapped the (I assume original) 29 tooth speedo gear for a 26 tooth gear as per the chart posted on this site. I got the gear from eBay and verified that it has 26 teeth. With the 29 tooth gear and 32-inch tires, the Jeep was around 5 to 6 mph fast compared to actual speed. Now, with the 26 tooth gear and 32-inch tires, the Jeep is 9 to 10 mph fast. I went for an 8.5 mile trip (according to the GPS app) and the trip odometer on the Jeep was off by about 0.15 miles.

Comparing ratios, the swap I did doesn't seem to make sense. For example (based on the first setup I had): 30 mph (Speedometer reading) / 25 mph (actual speed) = 1.2... 1.2 x 29 (original tooth count) = 34.8. This would mean I'd want a 35 tooth gear.

Does anybody have any idea what could be going on? Am I missing something here? I'm planning to swap to 4.10s when I locate the axles, so maybe it's not even worth chasing down right now.

Thanks
 
Well, since you counted 26 teeth on the speedo gear, then either you don't have 3.07 gears or you don't have 32" tires. But if any PO swapped the original 3.07 gears with something else and never removed the tag from the diff cover, then you may want to open the diff and start counting ring & pinion gear teeth.
 
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To verify R&P gear ratio you can jack up the vehicle (chock the front wheels) place a chalk mark on the drive shaft and tire; place the transmission in neutral and spin the tire. Count how many times the tire spins to how many times the drive shaft spins. This should help you determine what is installed in the differential.

You are going the wrong way with the speedo gear. Probably should be around 31.
 
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CharlesHS said:
To verify R&P gear ratio you can jack up the vehicle (chock the front wheels) place a chalk mark on the drive shaft and tire; place the transmission in neutral and spin the tire. Count how many times the tire spins to how many times the drive shaft spins. This should help you determine what is installed in the differential.

You are going the wrong way with the speedo gear. Probably should be around 31.
Click to expand...

Thanks for the advice. Could you give me your reasoning behind choosing a 31 tooth gear?
 
benny3577 said:
I know this issue has come up on the forum before, but I have tried some of the solutions and thought processes to remedy it and I'm still unsure. I am working on correcting the speedometer. My TJ has 3.07 gears and 32-inch tires. I have verified this ratio by checking the tags and turning the tire and watching the driveshaft spin.

I recently swapped the (I assume original) 29 tooth speedo gear for a 26 tooth gear as per the chart posted on this site. I got the gear from eBay and verified that it has 26 teeth. With the 29 tooth gear and 32-inch tires, the Jeep was around 5 to 6 mph fast compared to actual speed. Now, with the 26 tooth gear and 32-inch tires, the Jeep is 9 to 10 mph fast. I went for an 8.5 mile trip (according to the GPS app) and the trip odometer on the Jeep was off by about 0.15 miles.

Comparing ratios, the swap I did doesn't seem to make sense. For example (based on the first setup I had): 30 mph (Speedometer reading) / 25 mph (actual speed) = 1.2... 1.2 x 29 (original tooth count) = 34.8. This would mean I'd want a 35 tooth gear.

Does anybody have any idea what could be going on? Am I missing something here? I'm planning to swap to 4.10s when I locate the axles, so maybe it's not even worth chasing down right now.

Thanks
Click to expand...
The charts show you have the correct gear. So, if the correct gear is not giving the correct speed, then you have a different size tire or gear ratio.
 
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mrblaine said:
The charts show you have the correct gear. So, if the correct gear is not giving the correct speed, then you have a different size tire or gear ratio.
Click to expand...

Exactly what I was thinking....

benny3577 said:
Thanks for the advice. Could you give me your reasoning behind choosing a 31 tooth gear?
Click to expand...

IF you go from 29 to 26 speedometer gear numbers and the differences between speedometer and actual goes up then you are going the wrong way in speedometer gear ratio.
 
The speedometer is an analog gauge with a spring, it could be wrong. The odometer is digital, it isn't. GPS odometer readings are terrible, forget about those. You really need to decide if you care about the speedometer or odometer, they are two different things.
 
I've seen a number of speedo gear charts that don't jive with each other. I just installed a yellow box with the Jeep wiring harness included. I drilled a hole in the floor, installed a grommet, and ran the wires up into the center console. Using a GPS app on my iPhone, I verified the actual speed and calculated the correction factor. Programmed it in about 30 seconds and verified the speed to be bang-on the first time. When I change to my other set of tires, I'll work out the percentage. Tired of screwing around with the speedo sensor and wheels...

www.yellr.com
 
I realize this post is a few months old now, but I had a little more time to look things over. I re-checked that the gear ratio is 3.07 by jacking up the tires and counting driveshaft rotations. Also, I measured the diameter of the tires (when they were on the ground) and they were closer to 30.5 inches than 32 inches. I originally used a 26 tooth gear based on the assumption that the tires were 32 inches in diameter. Any ideas based on these findings? I’m thinking even if I went with a 28 tooth gear to match 30 inch tires, I’d likely be off considering that the old 29 tooth gear still made the speedo too fast and the new 26 made it worse.
 
If you are too err math impaired, post your current gear, your speed indicated by the speedometer at any speed along with the gps speed indicated at that speed from your phone and I'll tell you what the new speedometer gears will do.
 
pagrey said:
If you are too err math impaired, post your current gear, your speed indicated by the speedometer at any speed along with the gps speed indicated at that speed from your phone and I'll tell you what the new speedometer gears will do.
Click to expand...
I currently have 3.07s and a 26 tooth speedo gear. If it says I’m going 40 I’m going closer to 30.
 
When you installed the new speedometer gear did you verify the sensor numerical mark on the housing was indexed properly? There is a mark at the base and numbers 26-31 and 32-36 or something like that identifying where the sensor should be indexed to depending upon speedometer gear ratio so you get the proper speedometer readings. IF the indexing is correct the previous posts from other forum members applies.
 
benny3577 said:
I currently have 3.07s and a 26 tooth speedo gear. If it says I’m going 40 I’m going closer to 30.
Click to expand...
You really need exact numbers if you want to get it right the first time. Not closer to 30, the exact speed shown. What you posted suggests a 34 tooth gear and you have 4.10s but if you are off by a few mph so is the math.
 
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I've seen that chart, I'd bet the reality is the lower number is more like 40/32 making the gear about a 32 tooth and his ratio 3.73 it's all guesswork until you get exact numbers for the speeds.
 
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