Trying to figure out current gear ratio

[odoyle]

I need some help from the math wizzes out there or those with a similar situation. I have had my a LJ for a few months now and I have been trying to figure out why my speedometer is reading 10% faster than actual speed. I have 31" tires, 42rle auto, and 34 tooth speedo gear. I am guessing the previous owner must have re-geared it, but my question is what gear ratio?

 

[PJOJeep]

What size tire did the Jeep come with from the factory? Are the factory gear ratio tags still attached to the differentials? You can get pretty close to finding out what gear ration you have by jacking up the rear axle, setting it down on jack stands, and then mark tire & drive shaft, now rotate your tire slowly and count rotations of the drive shaft for 1 complete rotation of the wheel.

I have had my TJ since new in 2000, it came with 30” factory tires, I have 31” X 10.50” - 15’s on it now and my speedometer is pretty close to the Garmin GPS I have mounted on the dash. I ran it for about 5 years with 32” X 11.50 -15’s and was off about the 10% your referring to. I didn’t have a GPS back then, so the 10%

 

[PJOJeep]

Sorry, hit send. If you knew the size tires it was supplied with, you can calculate the difference in distance per revolution (tire manufactures usually list revolutions per mile). It is possible the PO regeared the speedometer...or the differentials, or it may just be larger tires.

 

[odoyle]

I assume it came from the factory with 30" tires also, the factory gear ratio would be 3.73. So with 31" tires my speedo should be actually be running a bit slow if it still had the factory gears.

Thanks for the feedback!

 

[glwood]

I assume it came from the factory with 30" tires also, the factory gear ratio would be 3.73. So with 31" tires my speedo should be actually be running a bit slow if it still had the factory gears.

Thanks for the feedback!

Since you have larger than stock tires, the only possibilities for the speedo reading faster than actual speed are the wrong speedo gear, or axles are regeared.

 

[odoyle]

It has the 34 tooth speedo gear which is what it would have come with stock. So I’m pretty sure it’s been regeared. I am hoping to find someone who has a formula to calculate the axle gear ratio.
I am looking to go up to 35” tires in future so I’m wondering if I may already have the correct gears.

 

[pagrey]

4.10s would do it, most LJ run pretty accurate on 31" tires. You can check the hard way by turning the wheels and counting revolutions but the math says 4.10/11s

And those would not be the right gears for 35" tires

 

[odoyle]

4.10 gears it is. I counted the drive shaft revolutions today. So I guess I may just go with 33"s when I replace my tires.