Question. I need to change my speedo gear due to my 33" tires. I have 2005 tj sport with 3.73 gears. I bought a 31 tooth speedo gear. I've installed it and I don't get anything. I lined the correct numbers up with the bolt hole like I seen on YouTube. I've seen on YouTube 2 videos one says line the numbers up to the bolt hole and the other says point the numbers down to the 6 o'clock position. I returned the first 31 tooth gear thinking it was bad. If I put the old gear back in it works but my speed is off. Now my question is which is correct?
2005 TJ speedometer gear install question
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I've never had to mess with the speedometer gear since mine is a Rubicon (which doesn't use a speedometer gear). @Jerry Bransford may be of some help on this one though.
The gear you bought wasn't bad, you just didn't rotate the speedometer gear housing quite enough for the new size gear to be able to engage its pinion gear down inside the transfer case. Aligning it like that video showed just got it close, not close enough. The speedometer gear housing is on a concentric so as you rotate it, it moves the speedometer gear in/out as needed for it to engage the speedometer gear.
A 31 tooth gear is the correct choice for 33" tires and 3.73.
Here's a website that might help... http://www.4x4xplor.com/speedo.html
TheGanzman
TJ Enthusiast
Since I just went through this on my '04, I might add that before you just ASSUME that your tires are "XX inches", actually MEASURE them at the inflated pressure you like while they are ON the vehicle! My BFG 31" tires are in reality closer to 30+ inches (wear, perhaps?), and with me being the perfectionist that I AM, I had to go with a 28 tooth gear instead of the 27 tooth gear that was suggested for my "31 inch" tires - now I am w/in 1.5MPH of actual speed on my speedometer, and my work here on Earth is DONE!
The circumference of the tire is what drives the gearing and ratio. Measuring a tire while it is mounted and on the ground is going to produce a lower diameter since the tire is slightly flattened on the bottom when it's on the ground... but the circumference hasn't changed from when the tire was still in the air before it was mounted. Measuring a tire's diameter while it's supporting the vehicle is going to give a misleading indication of its height for calculation purposes. Just going with the tire's sidewall diameter is going to produce very accurate results. My Rubicon's speedometer is dead-nuts accurate and agrees with my two GPS receivers at all speeds and all I put into my calculations for its Truespeed calibrator was its 35" diameter, not it's 34.73 diameter the manufacturer says it is.
Think about this... if your tire pressure is 20 psi or 35 psi, it still has the same rolling circumference yet you'd measure two different heights if you measured from the ground to the top of the tire at both of those two pressures. And it's how many revolutions the tires make per mile that counts which doesn't change at various tire pressures. A tire is not a balloon whose circumference changes with its pressure.
TheGanzman
TJ Enthusiast
Ha neither is mine. I miss the hell out of San Clemente, that's where I lived when I got married. Great town, I lived in those condos near the top of Pico which was a hell of a bike ride to get back up that hill!▲▲▲ - Well, I guess my work here on Earth is NOT done after all...
When I re-geared to 4.10's for my 33" tires almost all of the charts listed a 35 tooth gear. Turns out my speedometer was still off and I ended up swapping that out for a 36 tooth gear to get it right. I just can't remember now if it was reading fast or slow.
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Nep1303
New Member
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