Real Jeep Posted January 12, 2019 Share Posted January 12, 2019 Hi I am new to the forum. I have a 86 comanche with the 2.8l, ax5 transmission, NP207 transfer case and 3.54 gears. I bought this MJ as a project truck. Both my speedometer and tach are reading way off. I checked the speedometer gear and it is a 21 tooth gear. Is this the correct gear? Also the Rach is reading over 6000 when I know the motor is running just over 3000 ( I have checked it.) What could be causing it to read so much off. One last question, checking the vin number it indicates that this truck was built with the automatic transmission, but I can see no sign that it ever was. I have heard that the vin numbers were not that accurate in the last years of AMC. Thanks for any help I can get. Link to comment Share on other sites More sharing options...
Real Jeep Posted January 12, 2019 Author Share Posted January 12, 2019 Forgot to mention I am running 235/75/15 tires. Link to comment Share on other sites More sharing options...
87MJTIM Posted January 12, 2019 Share Posted January 12, 2019 You should probably have a 34 tooth gear. Search on “Al Gore’s Amazing Internet” for the proper gear for Jeeps with your tire size and diff ratio Link to comment Share on other sites More sharing options...
Pete M Posted January 12, 2019 Share Posted January 12, 2019 we had a link in the DIY section, but it looks like go-jeep's site changed so here's the updated link for speedo gear calcs: http://gojeep.willyshotrod.com/HowtoSpeedoGears.htm Link to comment Share on other sites More sharing options...
Green Mesa XJ Posted January 12, 2019 Share Posted January 12, 2019 Let me guess it’s a skinny green gear, the np207 has a reverse cut gm style gear, it’s not like the rest of the Jeep line 87-up. PiTA to find another gear or even the correct one for those tires. If you look at the sticker near the door of maybe in the glovebox or owners manual I think stock tires are 205 75 15 or 195 75 15, you running those 235 75 15 which is close to 30 9.5 15 I used to run on my XJ, and I was only off by 5mph, if you’re off more than this something else is wrong with that speedometer cluster Being as old as it is bad printed circuit (for the tach ) or water (damaging the speedo gears) from a windshield leak might be the culprit. Far as the tachometer if somebody swapped in a cluster out of a 4cyl model you rpm could be off. I thought there was a way to set them from 4cy to 6cyl, but I’ve never done this. Usually the 6cyl jeeps got the upgraded packages with the tachometers. If we assume this cluster is factory and correct for your mj I’d pull the cluster and clean the contacts to see if this helps. Far as the speedometer if you have cruse control I’d look to see if there’s a box the speedometer cable goes through, it’d be around the fire wall, the longer speedometer cable would go from the transfer case to this box then a shorter cable from this box the cluster. At least this is how cruse set up is on some of the older jeeps and other 80s vehicles. If that box is broken on the inside you’re speedometer might be off because of it. At least it was the cause for a vw I had years ago. Sent from my iPhone using Tapatalk Pro Link to comment Share on other sites More sharing options...
Rockfrog Posted January 12, 2019 Share Posted January 12, 2019 For that year range of vehicle, you have a fully mechanical speedometer, no wiring involved in it's function whatsoever.If you have a 21 spline gear in the speedo drive, that is your issue.235/75r15 and 3.54 gears are all you need to concern yourself with as far as the sppedo is concerned.You need to swap to a 33 o 34 tooth gear (according to most charts, as you technically split the range with a 235/75) on the sppedo drive housing. It will be a long shaft gear that you need. You may find a 31 tooth gear more accurate though (explained lower down). If you plan to swap axles/gearing/tires at any point, plan ahead and get the gear as well. If you need to, use your phone as a speedo for the time being, I've done this several times while mid build (Ulysses Speedo is my go to app for Android). Also take the speedo charts with a grain of salt, thy are made based on the factories tendency to read 10% under speed. I have actually found a 34 tooth gear to work perfectly for my MJ when I was 3.73 geared rolling on 255/75r17, and now use a 39 tooth with 4.56 gears and 33/13.50r15 with plans for 35's so the current 5% error (reads faster than actual) is fine for now and better than factory. As a result of this, I generally keep the speedo gears from all the cases I have had pass my way. As for the tach, there should b a potentiometer on the tach itself to adjust it. It is quite likely the whole cluster was swapped from a 4cyl vehicle. Sent from my SM-G930W8 using Tapatalk Link to comment Share on other sites More sharing options...
Green Mesa XJ Posted January 12, 2019 Share Posted January 12, 2019 Problem with the correct gear is it’s a np207, from what I remember all of the online speedo gear guides are meant for np231 or other more common transfer cases. The only place I found the correct gears was in a old parts catalogue but none of them were available like 15-20 years ago. It’s a very oddball gear to find. Part of the reason I’m upgrading to the np231 is to get a accurate speedometer, that and there’s just a ton of support for the 231 Sent from my iPhone using Tapatalk Pro Link to comment Share on other sites More sharing options...
Rockfrog Posted January 12, 2019 Share Posted January 12, 2019 Ouch, yeah, swapping to any 4cyl, 21 spline, 231 would solve that. And it'll likely come with a nearly correct gear. Sent from my SM-G930W8 using Tapatalk Link to comment Share on other sites More sharing options...
Real Jeep Posted January 13, 2019 Author Share Posted January 13, 2019 Thanks for the information and input guys. I am working offshore for a couple of weeks, but as soon as I get home I will be back to working on it. Link to comment Share on other sites More sharing options...
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