The Ins And Outs Of Speedometer AdjustmentPosted in How To: Transmission Drivetrain on July 1, 2012
If it were up to us, we wouldn’t even have a speedometer in a Jeep. You don’t need a speedometer off-road and many of our Jeeps feature a self-limited top speed that will keep us out of the sights of Johnny Law. Whether it is from chunked and out-of-balance tires, or gearing, or lack of power, we often find our comfort level on-road is somewhat slower than many posted speed limits.
That said, if your Jeep isn’t a clapped-out pile and it does see on-road time, you should pay some attention to your speedometer. In carbureted rigs, an out-of-tune speedometer can get you in trouble with the aforementioned copper. In a fuel-injected-rig you’ve got that issue as well as possible issues that an out-of-whack speedometer might cause with your fuel injection. The fuel-injection system takes information from the same place your speedometer does to determine a slew of things that help the computer figure out if the Jeep is running right. Parameters such as the fuel map, shift points, and check engine lights are but a few of the items that might be impacted.
If that weren’t enough, you are lying to yourself about your real mileage. Lifted and re-geared? You are putting fewer miles on your Jeep than you think. Lifted with bigger tires and stock gears? More miles than you think. This can affect your perception of everything from fuel economy to how often you do oil changes and more. It is simple, and in many cases cheap, to adjust your speedometer to read correctly. So follow along as we nail down the pertinent points of adjustment, no matter what Jeep you have.
Spicer 18 (’40-’71), Dana 20 (’62-’79), and Dana 300 (’80 only) (BW1305 and BW1339 ’73-’79)
In the speedometer adjustment game, the further back you go, the harder it can be to find parts, and the harder it is to get to them for swapping. Spicer 18 and Dana 20 T-cases have a speedometer drive gear (A) and a driven gear (B), which is a light press fit on the output shaft (C). The Dana 300 was the bridge between the old and new styles of speedometer gearing. If you’ve got a short-output unit from a ’80 Jeep, the gears can be hard to find, but most of the driven gears and speedometer drive gears that work with a Dana 20 or Spicer 18 will work. Herm The Overdrive Guy has both driven gears and drive gears and Quadratec has driven gears.
If you can’t find the driven gear and drive gear that you want, Performance Automotive and Transmission Center (PATC) is a great resource. Not only does the company offer speedometer head calibration, but it offers these ratio adjusters that go in line with your speedometer cable and aren’t that much more coin than a speedometer gear.
NP231 (’91 only) and NP242 (’91 only)
In 1991, things got weird again. Jeep switched over to the Bendix HO-style multiport fuel injection for the YJ and XJ/MJ. The Bendix system wanted to see some kind of VSS input. If it doesn’t see it, the check engine light comes on and the zombie apocalypse is unleashed. The big housing with two wires coming out of it is your clue that zombies might be in your T-case. We’ve seen ’92 Jeeps go both ways, so check before you go crazy figuring “Well Jp said I had a ’92 so I could use this.” This is kind of a bad news/ good news kind of thing. You can’t use the electric doohickeys we talk about elsewhere in this article, but it isn’t as weird as an ’87 YJ with an NP207. You can use the same gears we talk about above for the earlier NP231 and NP242. Seen in the foreground are 32-tooth and 38-tooth gears we got from Quadratec.
Dana 300 (’81-’86), NP208 (’80-’87), NP219 (’80-’82), NP228 and NP229 (’80-’84), NP231 (’88-’90), and NP242 (’87-’90)
Finally we hit normalization. Starting in 1981 the tailshaft on the Dana 300 grew to eliminate the driven gear setup. The output shaft grew an inch or two and the “gear” was cut right into the output shaft. Some claim that made the shaft weaker, but we don’t really know either way. What we do know is that if you have this aluminum elliptical speedometer gear holder, your life just got easier. Gears are widely available and there are many tooth counts to better fine-tune your speedometer’s reading.
Speedometer Gears (’41-’90 Jeeps)
If you have a Jeep made in 1990 or before, you have two basic speedometer adjustment options: either pull the speedometer out and take it to a speedometer shop to have it recalibrated or swap the speedometer drive gear in the tailshaft of your T-case. Of the two choices, we go with the speedometer gear every time. There was a time not too long ago that the only way to get a new speedometer drive gear was to visit the dealership and lie to them about what year, make, model, engine, and transmission you had. Different Jeeps over the years came with different axle gears, engines, and transmissions, and you would need to know which one had the right number of teeth for whatever gears and tires you were actually running. Today, it is easier. Call up either ATV Manufacturing or Quadratec, tell them the year and tooth count, and you are good to go. The readily available aftermarket gears are usually less expensive than getting the speedometer itself recalibrated.
NP231 (’92-’06), NP242 (’92-’06), and NP249 (’93-’98)
Then, in 1992 we saw another change in how the speedometer was driven. The speedometer itself was still electronic, but the sender changed a bit. This stubby black housing with three wires coming out of it replaced the longer silver housing that came before. The speedometer gears themselves also changed. They, too, became stubby. The loose gears shown here are 30-tooth and 37-tooth units from Quadratec. Quadratec also suggests new O-rings and at just $4.99 for the Mopar part, we agree. Speedometer gears are relatively inexpensive, but there are no half-tooth numbers or partial ratios, so if you have a slightly off-spec tire or out-of-whack gear ratio, they will get you close, but maybe not close enough. If you prefer more control, you can also adjust these electronically with a device such as the Superlift TruSpeed.
Some Jeeps don’t use any speedometer gear. Instead, the signal for the speedometer is generated with a tone ring and a magnetic pickup. You can have the dealership reprogram your speedo, or you can pick up a SpeedoHealer from Blue Monkey Motorsports for just a bit more than the price of two speedometer gears. It features plug-and-play installation on most Jeeps so easy, even a teenage girl can do it. It is waterproof, and offers -99.9 percent to 9,999.9 percent adjustment in 0.1 percent increments for the biggest range of adjustment with the finest incremental change available. If you wanted to, you could even readjust your speedometer for tire wear. It will work for all three-wire Jeeps, not just the Rubicon.
Aftermarket Electronic Speedometer
Most aftermarket electronic speedometers will run off of your Jeep three-wire sender, you just have to program them. We have this one running off the factory sending unit in a ’94 Wrangler with an NP231. For this AutoMeter Phantom electronic speedometer, press and hold the button, release, drive a measured two miles, press and hold the button, and then release. No numbers to deal with no dials to turn. Sure, the electronic speedometers are more money than the mechanical ones, but for any gear or tire change, an accurate speedometer is just two button pushes away with no gears to buy and nothing to swap.
The NP207 is such an odd duck, we didn't have any pictures of the speedometer gears. It was found in the YJ, and some MJs and XJs of that year(rare). However, PATC offers 18-through 22-tooth units for this oddball, although a ratio adjuster might be easier to find.
The JK ('07-and-up) Wrangler
Newer Jeeps no longer take any information about vehicle speed from the tailshaft of the T-case. With proliferation of next generation safety (read:nanny) systems, the on-board computer needs to know what each wheel is doing so that independent control of each is possible during extreme maneuvers. Gone are the days of just swapping a gear or putting a simple inline adjuster in the Jeep. Your dealership could recalibrate your speedometer for you, but depending on what kind of a relationship you have with them, this can be anything from free to really expensive - not to mention all the time wasted waiting in a dealer. This applies to '99-presnet Grand Cherokees as well as the above mentioned Wranglers. Hands-down, our favorite method of adjustment comes in the form of an aftermarket programmer. There is one available for just about any budget, no waiting in lines, and you can recalibrate as often as you need which is great for tire or gear swaps. Check with your manufacturer to see if the programmer will work on a Grand. There are numerous programmers out there for your JK that will allow you to adjust your speedometer (and so much more). Here are some of the ones we know of:
When we think GPS, we think of the slow responding navigation systems that we have on our windshield or in our dash. We can be accelerating for 3 to 5 seconds before they respond. This GPS Speedometer Interface from AutoMeter is not at all slow, and is said to work with all electronic speedometers. That means, boys and girls, that if you wanted to drive your existing (even factory) electronic speedometer off of it and never change or adjust anything ever again, this is your baby. It is highly water resistant, but shouldn’t be submerged. It comes calibrated for an AutoMeter electronic speedometer out of the box, but can be recalibrated very easily if your speedometer is off after you hook it up.