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Old April 12th, 2003, 18:07
Ivan's Avatar
Ivan Ivan is offline
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Join Date: May 2002
Location: Greensboro, Ga
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Real Life Speedo Gear Input Needed...

Getting ready for the axle re-gear, new tire thing finally on my '98. Of course I'm going to fix the speedo by regearing it as well. I've seen all the sites, but I'm torn between 2 gear choices of either 37 or 39 tooth since there is no 38 tooth option available. Based on the 32" BFG's I'll be getting which are 31.8" in diameter and 4.56 gearing (stock was 28.3 tire dia with 3.07's); Go Geeping site says 38 tooth, Jeeping.com site say 40 tooth (?), Cherokee America site says 37 tooth, JU Jeep tech site says 37 tooth also.

With my specs of 32's, 4.56 gears and a 5spd; what are the real world experiences that provide the most accurate speedo reads? Would it be either the 37 tooth or 39 tooth speedp gear option? Thanks!

Ivan
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Old April 12th, 2003, 19:35
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MaXJohnson MaXJohnson is offline
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I don't have the same gear/tire that you are planning on running, but last time I regeared, I changed speedo teeth proportional to the % change in gearing and it worked out perfect.

In your case, for a given road speed, you are increasing driveshaft RPM by 48.5% by changing the 3.07's to the 4.56 gears. At the same time, your are reducing driveshaft RPM with the larger tire by 11%. You end up with 132% of the original shaft RPM for your speedo gear to translate into MPH.

If my math is correct, you need to reduce the new shaft speed by 25% to return to your original speedometer reading for a given road speed.

Therefore, multiply your existing tooth count by .75 to determine the new speedo gear size.

or I could be full of it
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Old April 12th, 2003, 20:17
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DurangoXJ DurangoXJ is offline
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I went through the exact same thing with my 88 xj.
I went to the 32's, 4.56s, with the 5 spd tranny and the 37 tooth gear is what I used.
I got the info from the Cherokee America page:
http://www.off-road.com/jeep/cheroke...do/speedo.html

It has worked great for me and the speedometer is just about right

DurangoXJ
member #432
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Old April 13th, 2003, 01:30
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Gojeep Gojeep is offline
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My chart which you refer to is based on BFG's as I found them to be the most used. There was a lot of people saying my chart was not dead on a few years ago when it was based on stated height of the tyre. The chart is now based on BFG's that people have checked with a GPS as much as possible and is update it any time someone tests a different gear/ BFG tyre setup with a GPS.
So feel free to double check it if using the above combo but have had no complaints in the last 12 months or more
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