Jacob Ochs Posted November 19, 2014 Share Posted November 19, 2014 This question is asked a lot but mine has a little bit of a twist to it. Recently I have put a lift on my MJ 6.5 inches or so and put 33s on it! Now obviously the speedometer is going to be off because of a different geometry through the tires! So my speedometer reads 7Mph too slow exactly! I know in a model XJ or MJ to fix this issue you would just change the tooth size of the speedo gear! My problem is that I have the 86 MJ with the 2.8L and it uses a speedo cable instead of the electric pulse sending speedo! I was wondering how to correct this as I am doing a full gauge cluster swap this Friday. Thanks for any advice! Specs: 1986 Jeep Comanche 2.8L AX-5 transmission D30 front D35 rear both 4.10 gearing(d35 about to be replaced with Ford 8.8) 6.5 in lift and 33x12.50r15 Link to comment Share on other sites More sharing options...
HOrnbrod Posted November 19, 2014 Share Posted November 19, 2014 Doesn't matter, all electronic or cable speedometers all use a speedo gear for calibration based on tire size and drive ratio. Use this site to determine which gear you need: http://gojeep.willyshotrod.com/HowtoSpeedoGears.htm And what's with all the exclamation points! Link to comment Share on other sites More sharing options...
Jacob Ochs Posted November 19, 2014 Author Share Posted November 19, 2014 Doesn't matter, all electronic or cable speedometers all use a speedo gear for calibration based on tire size and drive ratio. Use this site to determine which gear you need: http://gojeep.willyshotrod.com/HowtoSpeedoGears.htm And what's with all the exclamation points! Thanks, helped a lot! And I just find it makes everything sound welcoming/nice IMO Link to comment Share on other sites More sharing options...
Eagle Posted November 19, 2014 Share Posted November 19, 2014 So my speedometer reads 7Mph too slow exactly! I know in a model XJ or MJ to fix this issue you would just change the tooth size of the speedo gear! manche 2.8L AX-5 transmission D30 front D35 rear both 4.10 gearing(d35 about to be replaced with Ford 8.8) 6.5 in lift and 33x12.50r15 Impossible. The differential is a percentage, so any difference increases proportionately to the speed. In other words, if the speedometer is off by 2 MPH at 30, it's off by 4 MPH at 60 and 6 MPH at 90. Link to comment Share on other sites More sharing options...
joester1908 Posted November 19, 2014 Share Posted November 19, 2014 my 90 has the cable too. They call it a "long shaft" speedo gear, and they call the electronic ones "short shaft" Link to comment Share on other sites More sharing options...
HOrnbrod Posted November 19, 2014 Share Posted November 19, 2014 The short shaft gears didn't happen until1993, so all MJs have the long shaft speedo gears, including the 91-92 models w. electronic speedos. Also, the gears for both are the same, and the shafts can be changed out easily. Link to comment Share on other sites More sharing options...
xjrev10 Posted November 20, 2014 Share Posted November 20, 2014 So my speedometer reads 7Mph too slow exactly! I know in a model XJ or MJ to fix this issue you would just change the tooth size of the speedo gear! manche 2.8L AX-5 transmission D30 front D35 rear both 4.10 gearing(d35 about to be replaced with Ford 8.8) 6.5 in lift and 33x12.50r15 Impossible. The differential is a percentage, so any difference increases proportionately to the speed. In other words, if the speedometer is off by 2 MPH at 30, it's off by 4 MPH at 60 and 6 MPH at 90. Correct. Was gonna post relatively the same thing.. Link to comment Share on other sites More sharing options...
Jacob Ochs Posted November 20, 2014 Author Share Posted November 20, 2014 So my speedometer reads 7Mph too slow exactly! I know in a model XJ or MJ to fix this issue you would just change the tooth size of the speedo gear! manche 2.8L AX-5 transmission D30 front D35 rear both 4.10 gearing(d35 about to be replaced with Ford 8.8) 6.5 in lift and 33x12.50r15 Impossible. The differential is a percentage, so any difference increases proportionately to the speed. In other words, if the speedometer is off by 2 MPH at 30, it's off by 4 MPH at 60 and 6 MPH at 90.Not impossible, I don't mean to argue but I know for (edit: almost) a fact it's 7mph I don't know if something was messed up before but it's 7mph off I've driven by a radar sign and when my speedo says 55 it says 62. Now I realize there could be some error! So I did it again and it read the same! The radar could be off but who knows I would just be happy if it worked again!:) Link to comment Share on other sites More sharing options...
xjrev10 Posted November 20, 2014 Share Posted November 20, 2014 You are not getting it. The faster you go, the more mph your speedo will be off. Link to comment Share on other sites More sharing options...
Rockfrog Posted November 20, 2014 Share Posted November 20, 2014 Exactly, I use the road side radars too, and at 100km/h my stock speedo was 10km/h off... 10% ... Meaning at 50 km/h I was actually doing 45km/h ... 10%. It is only 7mph out at that speed, not at all speeds. Link to comment Share on other sites More sharing options...
Eagle Posted November 20, 2014 Share Posted November 20, 2014 So my speedometer reads 7Mph too slow exactly! I know in a model XJ or MJ to fix this issue you would just change the tooth size of the speedo gear! manche 2.8L AX-5 transmission D30 front D35 rear both 4.10 gearing(d35 about to be replaced with Ford 8.8) 6.5 in lift and 33x12.50r15 Impossible. The differential is a percentage, so any difference increases proportionately to the speed. In other words, if the speedometer is off by 2 MPH at 30, it's off by 4 MPH at 60 and 6 MPH at 90.Not impossible, I don't mean to argue but I know for (edit: almost) a fact it's 7mph I don't know if something was messed up before but it's 7mph off I've driven by a radar sign and when my speedo says 55 it says 62. Now I realize there could be some error! So I did it again and it read the same! The radar could be off but who knows I would just be happy if it worked again! :) You can be off by 7 MPH when your speedo says 55. But that's the ONLY speed at which you'll be off by 7 MPH. When your speedo says 27-1/2, you'll be off by 3-1/2 MPH. It's a proportion, not a constant error. Link to comment Share on other sites More sharing options...
Jacob Ochs Posted November 20, 2014 Author Share Posted November 20, 2014 So my speedometer reads 7Mph too slow exactly! I know in a model XJ or MJ to fix this issue you would just change the tooth size of the speedo gear! manche 2.8L AX-5 transmission D30 front D35 rear both 4.10 gearing(d35 about to be replaced with Ford 8.8) 6.5 in lift and 33x12.50r15 Impossible. The differential is a percentage, so any difference increases proportionately to the speed. In other words, if the speedometer is off by 2 MPH at 30, it's off by 4 MPH at 60 and 6 MPH at 90.Not impossible, I don't mean to argue but I know for (edit: almost) a fact it's 7mph I don't know if something was messed up before but it's 7mph off I've driven by a radar sign and when my speedo says 55 it says 62. Now I realize there could be some error! So I did it again and it read the same! The radar could be off but who knows I would just be happy if it worked again! :) You can be off by 7 MPH when your speedo says 55. But that's the ONLY speed at which you'll be off by 7 MPH. When your speedo says 27-1/2, you'll be off by 3-1/2 MPH. It's a proportion, not a constant error. Okay that makes more sense! Thanks for explaining guys. I thought it was a constant. Link to comment Share on other sites More sharing options...
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