Installing larger tires on your Jeep will give you a few problems to overcome, including an incorrect speedometer. On a stock Jeep, the speedometer knows that when the tires turn one revolution you have gone so far, but when you put larger tires on, the speedometer still thinks that one revolution of the tire takes you the same distance.
This will cause your speedometer to display a speed slower than what you are going, to underreport your miles traveled, and may cause your computer / automatic transmission to shift at less than optimal times.
Luckily, it's easy to correct this on most Jeeps (Rubicon models will most likely need an electronic upgrade from the dealer). All you need to do is install a new speedometer gear in your transfer case. This gear will have a different number of teeth than your stock gear – the number of teeth on the new gear is determined by the size of your tires. In my case I installed 33 inch tires on my 2005 Wrangler TJ, so I needed a new 31 tooth speedometer gear. I picked one up at Quadratec for $40 (the gear looks like it should cost $4, but $40 is the cheapest I could find).
This new gear covers the following Jeeps:
Here is what the new gear looks like:
To install the new gear, first find your existing speedometer gear. It's near your rear driveshaft, where it goes into your transfer case. You'll see a wire going into it.
The speedometer is held in by one bolt, so just unbolt it and pull the speedometer out. Some oil may come out with the gear, so have a drop cloth / bucket under it, and protect your eyes.
This is the old speedometer gear (red) next to the new speedo gear (green).
When replacing the new speedometer gear, you have to make sure that it's pressed up against the teeth inside the transfer case. The way this is done is by rotating the housing until it's in the proper alignment. Once you get everything in the correct position, replace the single bolt.
Notice the different position of the old vs new speedometers
That's it! You should be able to hop in your jeep and have the speedometer work perfectly. In my case, I didn't rotate the speedo on my first try, and when I drove the Jeep, my speedometer didn't work at all. I simply loosened the bolt and rotated it by hand until I felt the gears mesh, then I was good to go.
This will cause your speedometer to display a speed slower than what you are going, to underreport your miles traveled, and may cause your computer / automatic transmission to shift at less than optimal times.
Luckily, it's easy to correct this on most Jeeps (Rubicon models will most likely need an electronic upgrade from the dealer). All you need to do is install a new speedometer gear in your transfer case. This gear will have a different number of teeth than your stock gear – the number of teeth on the new gear is determined by the size of your tires. In my case I installed 33 inch tires on my 2005 Wrangler TJ, so I needed a new 31 tooth speedometer gear. I picked one up at Quadratec for $40 (the gear looks like it should cost $4, but $40 is the cheapest I could find).
This new gear covers the following Jeeps:
- 93-01 Jeep Cherokee (XJ)
- 93-98 Jeep Grand Cherokee (ZJ)
- 97-06 Jeep Wrangler (TJ)
- 93-95 Jeep Wrangler (YJ)
- 04-06 Jeep Wrangler Unlimited (TJ)
Here is what the new gear looks like:
To install the new gear, first find your existing speedometer gear. It's near your rear driveshaft, where it goes into your transfer case. You'll see a wire going into it.
The speedometer is held in by one bolt, so just unbolt it and pull the speedometer out. Some oil may come out with the gear, so have a drop cloth / bucket under it, and protect your eyes.
This is the old speedometer gear (red) next to the new speedo gear (green).
When replacing the new speedometer gear, you have to make sure that it's pressed up against the teeth inside the transfer case. The way this is done is by rotating the housing until it's in the proper alignment. Once you get everything in the correct position, replace the single bolt.
Notice the different position of the old vs new speedometers
That's it! You should be able to hop in your jeep and have the speedometer work perfectly. In my case, I didn't rotate the speedo on my first try, and when I drove the Jeep, my speedometer didn't work at all. I simply loosened the bolt and rotated it by hand until I felt the gears mesh, then I was good to go.
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