Alright, i have an 86 mj, i know its not an xj but same difference. My problems are numerous, but id like to address my top speed, or lack there of. At 2500 rpms and in fifth gear im barely doin 55mph. I do not know what gears are in there now, the tags are broken off, not removed so its probably a stock gear ratio. Is there anyway to figure out my ratio with a mph vs. rpms equation? Or calculator? Any feedback would be awesome, maybe someone who has had the same problem and rectified it? On a personal note, i love my torque, dont get me wrong, but i need to use the expressway to goto my new job or else it takes me another 40 min to get there. Again, thank you all gor the help you have givin, and the help im sure to get with this.

 

my MJ had 3.08 gears in it, I did axle swaps and went to 3.55's I was running 30's but not pulling anything close to those RPMs at 55

 

If my math is right, and you're running stock sized tires, that works out to roughly 4.45 for the rear end. I don't know what options there are for the axle you have in there, but there's certainly rear ends that high for other applications.

 

I should probably add that i dont have the 4.0.. My truck came with a little 2.8 v6. I have to figure something out, or find a vehicle thats better with gas. Thanks for the help guys, ill get the actual gear size in the next few days. When it isnt 20° outside.

 

Also, they are stock 16's off of a 04 wrangler x. Stock tire size for that is whats on mine, which actually gave me a little bit better of a top end.

 

Revs per mile of stock 04 Wrangler tires are either 735, or 752 (roughly) depending on which stock size you have. Assuming 2500rpm puts you at 55, that puts the rear end at ~4.26 or ~4.36. Of course, since you've changed your overall tire diameter, your speedo is probably not even close to registering the proper speed.

All that said, if you jack up the rear end, put a mark on your drive shaft, and turn the rear tire one rotation, you can count how many times the drive shaft goes round, and get your rear end ratio that way.

 

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