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Speedometer Correction for different tires

M

muddybeast

NAXJA Forum User
Location
Benton, IL
Does anybody know how big of a deal it is to correct the speedo when you put on bigger tires? I read somewhere it is just a little gear onb the t-case and its about a 5minute job. Can anyone elaborate on this and what size gear would I need for 35inch tires? Just to let you know I got a speeding ticket and the cop said if I correct the speedo they will more than likely drop it. Thanks alot
 
Oh yeah I still have the stock gears. Whatever they are in the 1996 upcountry package without the towing kit. (yes i know I need gears and its on the list of things to do) Thanks again
 
Go to the dealership. Give them all the info on tire size, and gear ratio in the axles. They can look it up for you. Then when you get the gears, take the receipt to the court and they will most likely either drop the ticket or change it to a defective equiment ticket.
 
does anyone know the stock speedo gear size?
looks like it would be a 35 from the chart. i just put in 4.56 and 33's (=39 tooth)
wondering if changing it would be necessary
 
Stock speedo gear size is irrelevant; it will vary depending on the Jeep's stock gear ratio and I believe even tire size option from the factory. My 96 XJ w/3.55s had a 34-tooth stock, and a 39-tooth corrected the 32s and 4.56s to almost dead-on accurate.
 
ya, i know. i was figuring 3.55 and 215's stock
if i run a 35 tooth where i should be running 39 tooth. is my speedo going to read higher than actual or lower. i think it will be reading higher than actual, right?
 
Since you'd have the same gears and same size tire (at least according to the sidewall) a 39 should work for you, although I'd still time it against a couple measured distances so you know for sure once it's installed.
 
Do you have a GPS or know where you can borrow one for a couple of days?

There is a relatively easy way, using a GPS to get as close as you can get depending how you want you error to be. (You may find, like I did, that you need a ??.5 tooth gear. Then you have to pick whether you want a little slower or faster than real.)

You set the "trip meter" on the GPS to zero and the trip meter on your jeep to zero. (If you don't have a trip meter, just right down the odo reading.)

After you drive for a while, like 50-100 miles (make sure the GPS gets turned on and has "aquired" location each time you drive your jeep, if it's not one trip).

Then when the jeep cools down, pull the current speedo gear and count the teeth.

The equation is as follows:

New number of teeth = old number of teeth x (miles on odo / miles on GPS)

For me that was x = 37 x (90 / 100) or 37 x .9 or 33.4 I choose a 33 tooth gear, but I could have gone 34.

34 would have made my speedo read low, so I went 33.

(I think I got all this right, but a sanity check would be nice. I did this rather quickly from memory...)

bburge
 
LouisianaZJ said:
does anyone know the stock speedo gear size?
looks like it would be a 35 from the chart. i just put in 4.56 and 33's (=39 tooth)
wondering if changing it would be necessary
Click to expand...
im running the same gear and tire set up and the 39 tooth gear works perfect.
 
old_man said:
Go to the dealership. Give them all the info on tire size, and gear ratio in the axles. They can look it up for you. Then when you get the gears, take the receipt to the court and they will most likely either drop the ticket or change it to a defective equiment ticket.
Click to expand...
i tried this when i did mine and they had no clue which one i needed. they ended up getting me the wrong part the first time. your best bet is to research on the internet
 
Old thread, new prices. Using the table at the link above I called my local dealer with the part number for the drive gear: $92.34 USD plus tax :shocked:

I'm trying to figure if the better shift points will justify the hundred bucks. The XJ is 96 4.0 4x4 ATX +3" on 31" ATs/8" Wrangler alloys.

Will it actually drive better with the sensor corrected?
 
I recently corrected mine for the second time after I went from 31s to 33s the chart recommends a 31 tooth gear but the speedo shop only had a 30 tooth in stock and he said it should be close enough which it was. I used the GPS in my phone to verify the speed was correct, I have an iPhone 4 and yes there is a app for that. I verified the GPS speedo app using my mom's Jaguar and it was spot on so I figured it was legit. I then checked the speed in the jeep once I changed the gear and it was very accurate.

If you drive your Jeep everyday I would change it and save yourself a $300 speeding ticket. The faster you are driving the more the speed is off. In my Jeep when the speedometer said 45 it was actually doing 50 and when it said 70 it was actually around 78.
 
lasersnthjungle said:
Old thread, new prices. Using the table at the link above I called my local dealer with the part number for the drive gear: $92.34 USD plus tax :shocked:

I'm trying to figure if the better shift points will justify the hundred bucks. The XJ is 96 4.0 4x4 ATX +3" on 31" ATs/8" Wrangler alloys.

Will it actually drive better with the sensor corrected?
Click to expand...
partsmike.com is where I got mine for about $35ish a year or two ago.
 
For me it wasn't about the shift points, but more about the speedometer (mine was waaaay off) and getting proper odo readings.
 
Quadratec has speedo gears for $39.99 and they are quick to ship.
 
Looks like mine's off just over 6% which I understand translates to two teeth. The build sheet says the XJ was delivered with 225/75-15 so I'm thinking (?) it has a 34 tooth in it.

For whatever reason, the 32 is $54.99 at Quadratec (most of their others are $39.99), and Partsmike is $37.95.

Do I have to pull the old one and count teeth, or is it most likely the stocker is actually a 34?
 
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