Hello conrads! Bear with me, this is my first post. I recently completed a 1985 XJ with a 2.8L auto build, and dropped a I6 with a 5-speed with the NP231AMC in it from an 88 Comanche. I left the front and rear axles in the xj, which claim to be 3.55's. The Instrument cluster came from the comanche. Now here is my question. At 1900 RPM in (O)D My Speedo shows 64 MPH, but I'm really going 54/55 MPH. I went through all the gear ratio calculators online, but nothing really makes sense. It basically computes to around 4.10 gear ratio. Did they ever put the 4.10 in the 85 xj other than with the 2.5L?

 

Welcome to CF!

I would assume It has 4.10's, the 2.8 with an automatic couldn't get out of its own way, so I'm sure they would have put taller gears in it to get some torque back. Just my guess, so i could be wrong.

 

I also built an 85 XJ that had the 2.8. I swapped in a SBC350/TH350 and kept the axles. During my research I'm sure I read that the original gear ratios are 4.10s. My speedo is way off too. I can only drive about 50-55 mph down the road and I'm taching at about 3K. My speedo only reads about 40-45. Need some new axles.

 

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I also built an 85 XJ that had the 2.8. I swapped in a SBC350/TH350 and kept the axles. During my research I'm sure I read that the original gear ratios are 4.10s. My speedo is way off too. I can only drive about 50-55 mph down the road and I'm taching at about 3K. My speedo only reads about 40-45. Need some new axles

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Yeah, i heard that the worm gear in the NP231 for the speedo cable is transferable per part number, cluster type, and spline. I haven't gotten that far into it yet, but I do know I will be researching it. As far as axles go, I have 4.10's and 3.55's Front and rear available, but just don't want to go through all of that if I don't need to, especially since I don't know which ones I have. I guess I'll have to get it up on the lift and do it the old school way... rotate the tires and count the driveshaft. This XJ only has 46K on the body and axles, and I honestly would like to preserve the 'Lack of use' that it has. Thanks for the replies.

 

Well, after a long day of searching on the internet and panning through all of the donor jeeps I have access too, I pulled a worm gear out of another NP231 from another fellow cherokee w/ 3.55's in it. and exchanged it with the one that I had. Yellow(old per 4.10's) for Orange (new per 3.55's). Finally Speedo matches GPS and I'm on my way. Thanks for all the comments