speedometer gear question

z50k2 · #1 08-13-2006, 04:55 PM

Here's my problem.
I just put a 2004 wrangler together, and cant' get the speedo to dispaly the right speed. I think the main reason for this, is that I am using a Rubicon computer with a sport t'case.
The rubicon takes electrical info from the t'case by the way of a sensor, where the sport uses the traditional speedo gear for this.
I'd like to get the right gear the first time, instead of buying all of the gears available.
Here is my data from the 30 tooth, and 34 tooth gears.
65 on the speedo is 25 mph 34 tooth gear
80 on the speedo is 31 mph 34 tooth gear
65 on the speedo is 22 mph 30 tooth gear
80 on the speedo is 27 mph 30 tooth gear
They make as high as 46 tooth, I think. I'm sure this is algebra, but I was sleeping that semester, and the semester after that..
Anyone?
Thanks, Ricky

Jim_Lou · #2 08-13-2006, 06:32 PM

Easy. It's a matter of logic, not algebra.

With the 34-tooth gear you're off by a factor of 2.60 (65/25) at 25mph, 2.58 @ 31 mph (80/31).

With the 31-tooth gear you're off by a factor of 2.95 @ 22 mph (65/22), 2.96 at 27 mph (80/27).

That the readings aren't linear and consistent is not at all unusual, in my experience. Don't let it bother you.

With the smaller gear, the speedo gets less accurate, so you need a much larger gear.

Now, with a 34-tooth gear, we're off by about 2.6 and we need a bigger gear. So get one that's 2.6 times as big as a 34, or an 88.4-tooth gear.

And to confirm it, with a 31-tooth gear you're off by about 2.95, so a gear 2.95 times as big as a 31 would have 91.5 teeth.

It looks like you need a gear with 89 or 90 teeth, or twice as big as the largest they make. You can get a 45-tooth gear and mentally divide the speed and odo reading in half. I have no idea what will happen when the speedo tries to go past its maximum reading. A speedometer shop might have a 2/1 gearbox. That and a 45-tooth gear will get you real close.

Also, a speedometer shop might be able to electronically calibrate the speedo for you. I put a Stewart-Warner electronic in my Jeep. It had a pickup that screws into the standard speedo drive nipple on the transfer case. Then it has a bunch of DIP switches on the back that you set to calibrate to the speedo drive gear, axel ratio and tire size.

After two of them died I got a VDO electronic. It uses the signal from the SW sender, but has a calibration button on the face. You hold it in, turn the ignition on and start the engine. Release the buitton, drive exactly one mile and push the button again. Voila, it's calibrated!

LegendaryJeep · #3 08-14-2006, 12:08 AM

Another option, and perhaps the more geeky and expensive, but rather cool none the less, is to get the Nordskog digital GPS speedometer.

Nothing like having the eye in the sky tell you how fast you're going.

http://www.nordskogperformance.net/p...l/hga02160.htm

Come to think of it... I may get this little beast myself, and set fire to my speedo gears and cables. Sure is a nice headache cure.

LEVE · #4 08-14-2006, 07:58 AM

The speedo-gear change ain't going to do what you need. The answer to the question is not a cheap one. You're best served by buying a Truspeed electronic speedo calibrator.

MTPockets · #5 08-14-2006, 08:42 AM

Dakota Digital makes a converter box that is cheaper than the Truspeed unit. Not sure it will work for your application, but it is worth taking a gander at.

http://www.dakotadigital.com/index.cfm/p...prod/prd126.htm

z50k2 · #6 08-14-2006, 12:35 PM

Thanks for the input guys, I got it figured out.
I read somewhere that a Rubicon speedo cluster was different that a regular sport or unlimited cluster. This is wrong...
The speedo clusters (for at least 2004) are all the same.
I know this by swapping out my spares for this jeep, and none had any difference.
Myth solved...
Now, the problem with the speedometer.
I have 2 2004's, so I started playing with parts, and it turns out that the computer I was using, wouldn't read from my gear driven sender properly. I swapped computers, and the speedo is now reading correctly.
I think that when my jeep was sitting in the wrecking yard, someone swapped in an older computer, that ran the engine, but didn't work the speedo correctly.
I just found another computer online, and hopefully it will work..
Thanks again,
Ricky

Pablo · #7 08-14-2006, 01:12 PM

This is good news, as I was going to tell you what I had to do.
First I had to get a gear from a 1990's cherokee, then I got a gear from a 2001 Dodge truck, I then called advance adapters and had them make me an inner conversion gear that transforms the gearing from one gear to another and then back. I know this sounds crazy, but it worked. I can now see how fast(or sloww) I am going.
Happy Trails.

CalJeep · #8 03-14-2007, 03:22 PM

http://www.go.jeep-xj.info/HowtoSpeedoGears.htm

Changing Your Jeep Speedometer Gear

The second link above is the best I've found, I did exactly as it read and fixed my speedometer in 5 minutes after getting the right gear from 4wd.com for 24.99...
I transmission shop and my local jeep dealer were quoting $250.00 plus for the job...

Good luck...it's really easy one wrench and 5 minutes..