TJ Speed Gear Issues

XCRN

TJ Enthusiast
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Jun 1, 2020
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469
Location
Springfield OH
I recently finished swapping my rear axle from a Dana 35 with 3.08 gears to a Dana 44 with 3.73 gears. I have 245/75R16 tires so 30.5”. Referencing the numerous charts online, they all said to use a 33 tooth gear for the VSS. Swap it in and my MPH is reading 5mph over the GPS verified speed. I ensured I had the VSS clocked correctly to show 32-38 at the 5-o’clock position and verified the gears are in-fact 3.73 by counting pinion to tire rotations. Prior to the axle swap I the speedo was reading very slightly slower than actual speed since I had smaller tires from the factory.

So I am at a loss on what my issue might be. Any suggestions?

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I have a 34-tooth gear in mine, with 3.73 and 31x10.50-15 tires (actual size about 30.5"). It's dead-on accurate with a GPS, most accurate speedo I've ever seen.

But now I'm changing tire size, and soon will be re-gearing, so I'll be hopping into your boat, trying to get the right gear.
 
I have a 34-tooth gear in mine, with 3.73 and 31x10.50-15 tires (actual size about 30.5"). It's dead-on accurate with a GPS, most accurate speedo I've ever seen.

But now I'm changing tire size, and soon will be re-gearing, so I'll be hopping into your boat, trying to get the right gear.

I wonder if a single tootj difference can affect the speedo by 5mph?
 
So maybe someone could figure out what gear I need to get with the starting point being the 33 that is supposedly correct but is not. At 60 reading on the Jeep speedo, I am going 56mph on the GPS. To reiterate, 30.5” tires, 3.73 gears. So if the 33 is not it, what gear should get me closer to accurate?
 
I just finished a similar adjustment. It is not uncommon for tires to run a little small in diameter, especially BFG-my 33x12.50s are more like 32”. This makes the Speedo show too fast. To slow the Speedo down to your gps value you need more teeth on the gear. I would use:
33 teeth X 60/56 = 35.35 teeth. 35 teeth will raise the actual (gps) speed to 59.4 (35/33 X 56) when the Speedo reads 60. 36 will be 61 Actual at 60 indicated. If it was mine I’d try a 35 tooth gear.
Good luck,
TJBUD
 
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I just finished a similar adjustment. It is not uncommon for tires to run a little small in diameter, especially BFG-my 33x12.50s are more like 32”. This makes the Speedo show too fast. To slow the Speedo down to your gps value you need more teeth on the gear. I would use:
33 teeth X 60/56 = 35.35 teeth. 35 teeth will raise the actual (gps) speed to 59.4 (35/33 X 56) when the Speedo reads 60. 36 will be 61 Actual at 60 indicated. If it was mine I’d try a 35 tooth gear.
Good luck,
TJBUD

Perfect, I knew there was some kind of maths involved. Ill try a 35 and see what happens.
 
This is why i always tell folks to not go off of a chart, and just do the math to see how far off they are. Then they know exactly how many teeth to add or subtract to adjust.
 
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Got lucky and had a spare 231J I had lying around my parts pile, no idea what it was from and it happened to have a 35 tooth gear in it. Threw it in and is about right on the money according to GPS. So I can return the one I just ordered when it arrives.
 
I just finished a similar adjustment. It is not uncommon for tires to run a little small in diameter, especially BFG-my 33x12.50s are more like 32”. This makes the Speedo show too fast. To slow the Speedo down to your gps value you need more teeth on the gear. I would use:
33 teeth X 60/56 = 35.35 teeth. 35 teeth will raise the actual (gps) speed to 59.4 (35/33 X 56) when the Speedo reads 60. 36 will be 61 Actual at 60 indicated. If it was mine I’d try a 35 tooth gear.
Good luck,
TJBUD

E2D4B250-C208-4D32-91B8-D8EC5010F804.png
 
Got lucky and had a spare 231J I had lying around my parts pile, no idea what it was from and it happened to have a 35 tooth gear in it. Threw it in and is about right on the money according to GPS. So I can return the one I just ordered when it arrives.

This thread inspired me to go check my junk pile too,lol. Found a 35 tooth gear thats within 1mph or so with my 4.10/33s 😄