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The Ins And Outs Of Speedometer Adjustment

Classic Tach Gauge
Pete Trasborg
| Brand Manager, Jp
Posted July 1, 2012

Speed How To

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.


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