When you put bigger tires on your Jeep, the stealership will tell you that you need to get the computer reprogrammed for $50 or more. But that isn't really true for non-Rubicon owners. You can purchase a $10 speedometer gear and replace it yourself in about ten minutes and get your speedometer and odometer working correctly again. Here's how...

Changing Your Speedometer Gear

One day, you might change the size of the tires on your Jeep. When you do, you will find that on your next drive around town that your speedometer is now incorrect. Worse - if you have an automatic transmission, your shift points will be incorrect, and the transmission will not perform up to specifications.

Jeep Wranglers and Rubicons both get their speed readings sent to the computer from senders in the transfer case. The Rubicons transfer case is different from the Wranglers, as it uses an electronic sender. When you get bigger tires for your Jeep Rubicon, you have to have the on board computer reprogrammed to hold a different set of variables with which to calculate speed.

If you have any other model of Jeep Wrangler, SE, X, Sahara, or Sport, then you have the El Cheapo transfer case, the NP231J, and it uses a highly sophisticated scientific system which is very complex with which to calculate your speed: A little plastic gear on a stick.

When NP231J owners buy bigger tires, they can simply buy a new speedometer gear, install it in the transfer case in under 30 minutes, and their automatic transmission and speed readouts will be corrected. That's all that is necessary.

You are learning this the easy way. I learned it the hard way.

Jeep Service Techs Need Reprogramming

My brakes were pulling to the right a little. I decided to take my Jeep in to the stealership and see if they could replace a brake line under warranty. Fat chance of that happening. As expected, they claimed they could not replicate the problem. While there, I asked them about having switched from 28" tires (P225/75/R15) to 30" tires, and what I needed to do to get the speedometer readout corrected.

The technician told me "You need to have the computer settings changed." He quoted $50 for the job. I passed on it, thinking I would call around to other dealerships and get a better deal than the appalling price of $50 for 30 seconds of hooking an electronic gizmo to the computer connector under the dash and clicking buttons.

I called another stealership for a price check: $84.00. Another: $42.50. And a fourth: $78.00. I asked each of them in turn if I could just get a speedo gear for my automatic, and they each told me confidently "No, you don't need to do that. We reprogram the computer for you. That's all it needs." Yeah, that's all. Just $50 or more to have someone do absolutely no labor at all vs. my doing 30 minutes of what is a reportedly a super-easy installation.

Finally, I called a fifth dealership, and there a service technician named Sam answered the phone. "Sam? I'm wondering about having upsized my tires and what I need to do to get the speedometer readings and shift points corrected. I've read online I only need a speedo gear swapped out that I can do myself. But I've called four dealerships and they all say I need the computer reprogrammed. I have an NP231 transfer case, not the electronic one in the Rubicon."

Sam paused, and then he said, and I was surprised to hear this, "Heck, I don't know. You are talking way over my head about it. You know more than I do. Let me ask someone, just a sec." In a minute, he came back on the phone, "Hey buddy, you don't need any reprogramming for that transfer case. Just get a speedo gear. They're cheap. In fact, go to Speedometer Service Company in Roswell, GA, and they have them there. Their number is 770-569-8255."

Sam not only admitted he didn't know, he asked for me, told me the truth, and then sent me to the best place to get a speedo gear. And he was polite. Sam, if you go to Hell when you die, it won't be because of this. Your mama raised you right, unlike the other service technicians I spoke with.

Speedometer Service Company

My new friend Sam pointed me to Speedometer Service Company of Roswell, GA. I drove to their office, on Mansell Lane just off of Highway 9 in North Roswell. It was a non-descript small, industrial building with tan bricks and an almost unmarked front entrance. I went in, and there was a nice receptionist who pointed me to the parts counter. Back I went.

Inside, I asked them for a speedo gear and gave them my part number, calculated from the matrix that is always recommended on JeepForum.com by the experts. I have a miniature copy of it here too on my Novice Guide to Tires and Wheels. My rear axle is a Dana 35 with 3.73 gears in it. My tires, after my upgrade, were 30 inches in diameter. I measured them just to be sure, because often a tire will say it is 30 or 33 inches, and then when you pull out the measuring tape, its really an inch or so smaller than that. Goodyear GSA's seem to be true to their claim - a real 30 inch tire.

That pointed me to the speedometer gear with 34 teeth in it. That's part number 52067634. The last two digits are always the number of teeth you need.

I had read in many places that getting a speedo gear could cost as much as $25. Speedometer Service Company had it for $9.36 including tax. I paid my money, they were very polite, and while they prepared my receipt, I looked at the other stuff they had out and chatted with them for a little while. They mostly do sales to dealerships. They work on all sorts of stuff. See their brochure which they handed to me across the counter.

The most interesting thing they have is a room in which you drive your car on rollers and they test the speedometer to check its accuracy. The cost of the test they said was $25. I didn't do this, since I know in my home town the police always have one of those big billboards with a radar gun attached to it telling you how fast you are going. I would drive past one on the way to work the next day, so no need. However, I'm tempted to do it anyway just to do it. It sounds fun!

Which Speedo Gear to Get?

The question is, which speedometer gear to get? The below chart should be some help to you. The part number is 520676 plus the two digits in the matrix next to your axle ratio and your tire size. The two digits are also the number of teeth that the gear has in it. I haven't included all gears or tires here. Who knows when to stop? This page mostly is help for novices, so someone who is getting 44" tires installed wouldn't be reading this wondering what gears to get from someone like me. I don't even have big wheels on my Jeep. But here is the help for you, nonetheless.

This table of gears is of no use to Rubicon owners. If you own a Rubicon, the transfer case does not use a gear to determine speed. Intead, it has a pure electronic sensor, and you must have the computer reprogrammed by the dealership in order to have your speedometer readout corrected after you up or down size your tires. In Winter of 2005, I called four local stealerships in my area to have this priced. The price for reprogramming ranged from $42.50 to $87.00. The lowest price was from a Chrysler dealership that does not sell Jeeps. Yes, they can work on your Jeep, too.

Required Tools

I now had everything that I needed to do the swap. You will need the following:

3/8 inch ratchet wrench.

13mm socket. You will use this a lot on a Jeep Wrangler TJ. Half the hex nuts on the thing are 13mm.

Automatic Transmission Fluid ATF+4. In a 2004 Jeep Wrangler TJ, you do not use ATF+3. I get this from the dealer for $5 per quart. You only need a little bit.

Bolt Size & Torque Specifications

Bolt	Size	Torque
Speedometer Gear Retaining Ring Bolt	13mm	90 ft-lb (seems excessive)

How to Switch Your Speedo Gear

This is not a difficult thing to do to a Jeep. Don't be nervous, this is an easy one. At a high level, let's review what we are going to do. We will pop out the speedometer cable, unscrew the bracket that holds the gear in, pull out the gear assembly, pull off the old gear, lube up the new gear, stick it on, and stick the assembly back in. Then we bolt it back on and we are good to go. It's not hard.

There is a picture of the speedometer gear I bought sitting on my kitchen floor. The 34 tooth gear is apparently green. It's just a simple plastic gear on a little metal shaft. That's it.

1. Prepare the area. No need to jack up the Jeep for this one. But it's a good idea to spread newspapers under your transfer case area to catch any tranny fluid that comes out of the case when you pop out the gear. When I pulled mine out, the gear dripped a little, and I was glad I had the paper down.

2. Crawl under your Jeep. I found this easiest on my back, head to the rear on the driver's side. I'm right handed. YMMV.

I took this picture while laying on my back. I bet this job is easier when you aren't trying to photograph yourself doing it! The assembly is highlighted in the photo.

3. Toggle the red snap lock on the speedo cable. See the red thing on the speedometer cable? That's a toggle lock. You have to push it in to unlock the cable from the assembly before you can disconnect it. In the photo above, the lock has been pushed down to unlock the cable. Then just push on the black raised part to release it and pull it out. It should come right off.

4. Pull off the speedometer cable. This will give you some room to work. Here you see mine hanging free.

5. Unbolt the retaining bracket. There is a retaining ring bracket that is holding the assembly to the transfer case. Pull this off, and the whole thing will pull free. Notice how it is put on so you can put it back together later. In the same picture above, I highlighted the retaining ring bracket and pointed to the bolt you will undo. The Factory Service Manual recommends huge torque on this bolt. I found mine was quite easy to remove and not torqued to those specifications. Once removed, place the bolt and the ring somewhere save and clean. You will need them again in perfect condition.

6. Remove the assembly from the transfer case. There is a rubber ring around the assembly just inside the socket that it fits into inside the transfer case. So, when you pull gently, it doesn't move, and when you pull hard, POP! It comes out pretty fast all of a sudden. I usually like to wear gloves for this sort of thing because being clumsy, my hands tend to get beat up. I didn't wear them last night, obviously. My assembly came out and left the speedo gear behind! LOL!

Isn't that always what happens when you are following directions? Something unexpected that you never would have thought of that didn't happen to the guy who wrote the book. Well, I ended up fishing around in the hole for the speedo gear and finally pulling it out. So much for the pristine environment inside my transfer case.

After I got out the gear, I laid it, the assembly, and the new gear next to each other and snapped another photo for comparison. They look nearly the same. The stock gear is red, and it has the number 36 on it for 36 teeth. It's the gear for P225/75R15 tires that came on my Jeep.

7. Coat the new speedo gear in some ATF+4. My transfer case uses ATF+4, so I picked up a bottle and poured a couple of ounches over the speedo gear with a little dish underneath. Once it was lubed up, I put it on the assembly. I also used some ATF+4 to lube the o-ring around the edges of the assembly where it seals up once back inside the transfer case. I used ATF+4 for my 2004 TJ, but you should check to see what yours needs. Automatic Transmission Fluid is sometimes incompatible, and it is necessary to use what you have in your transfer case already.

8. Stick the new gear into the assembly. Yep, you just stick it in there all the way and it's put together.

9. Replace the assembly in the transfer case. This is a little confusing. If you hold the assembly in your hand and examine it, you will see some numbers on it. They are ranges of gear teeth. On mine, one range was 39-45, and the other was 30-38 or something like that. The whole assembly twists to make room for bigger and bigger gears with more teeth in them. So, when you put the thing back together, you have to make sure the number range for your gear is facing more toward the transfer case and downish. Twist the assembly (it rotates so that the second bolt points in different directions relative to the bracket retainer bolt to make this work. This will help you get it back in.

If you think about it, this makes sense. If you were going to put in a gear that had 44 teeth which fit the same size companion gear inside the transfer case, to keep the teeth a size that would fit spaced right, you have to make the gear bigger. At some point, the gear is too big to go in the same place as the smaller one, so you have to move the offset of the little shaft so it can rotate and touch the companion gear. Twisting the assembly will do this for you.

Here's a little secret. I had no idea about this. My first and last gear were in the same range as each other, and I just shoved it in there until it took, made sure everything was perfectly flush, and screwed the bolt and retainer bracket back together, connected the cable, and performed a test drive. It worked, and I figured I was done. I was lucky, I guess.

10. Replace retaining ring. Screw back in the bolt. The FSM says to put it in with 90 ft-lbs of torque or more. I just tightened it a lot with a regular ratchet wrench. It didn't have any 90 pounds of torque on it before, so I think it will be fine.

11. Replace the speedo cable. Stick the cable back on, and snap the red lock back into place. Now test drive it.

All done!

Oh no! What Did I Do Wrong?!?r

Speedometer reads 0 mph all the time. Ask yourself some questions:

1. Did you connect the cable back up and lock it with the red snap ring?

2. Did you twist the assembly and point the little numbers in the right place? Maybe the gear isn't touching.

3. Did you put everything back together so it was flush as it was when you found it? Nothing should be sticking up or have space. The retaining bracket fits neatly flush on the t-case. Maybe you didn't stick it in all the way - and maybe that is because of question #2 being answered with "No."

Speedometer is still wrong! Maybe you bought the wrong gear, or maybe one of the reference charts you used as the wrong gear. Or, it is possible your tires need to be measured for diameter. Not all 30 inch tires are 30 inches across. Some are only 29 inches across. Some are 31 inches across. Measure the tire, get the correct gear.

Speed is not perfect! No speedometer gear will make your speedometer perfect. Speedometers are based on a level of technology from 50 years ago. Now, we have GPS and computers that test more accurately. Most speedometers are wrong - but only by 1 or 2 percent. And, don't be surprised when you find out that most odometers measure distance differently from speedometers. Many find their speedometer is wrong but the odometer is right, and when they fix the odometer, the speedometer is slightly off! Hilarious! Don't be such a perfectionist. It's OK for it to be slightly wrong. If you are doing 46 when it says 45, then that's good enough for government work. So, chill out, drop the top, take a ride in the country, and remember "It's a Jeep."

If you are still having problems, search for an answer at JeepForum.com. If the search doesn't turn anything up, try the advanced search. If nothing there, post a polite message in the TJ forum, and be nice, and be patient. Someone will answer.