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  #1  
Old September 8th, 2012, 20:58
ScienceguyXJ ScienceguyXJ is offline
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Speedo gear replacement to correct for bigger tires and gears

Running 37" tires with 5.38 gears and my speedo is reading over by 10%. I measured it by driving 10 miles based off of mile markers and setting my trip meter then doing some math to see how far it was off. I'm wondering if anyone knows how to figure out how many teeth I need on my speedometer gear in my transfer case to get my speedo/odometer back to reading correctly. Did this once before on another XJ with 33s and 4.56s that was also off by 10% I believe and called a performance shop and I think they told me I just needed a gear with 10 more teeth. Does that sound right to anyone? Thanks for any insight.
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Old September 8th, 2012, 21:17
ScienceguyXJ ScienceguyXJ is offline
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Re: Speedo gear replacement to correct for bigger tires and gears

http://jeep-xj.info/HowtoSpeedoGears.htm

Unfortunately this doesn't go up to 5.38 gears and doesn't include 37" tires. Anyone seen any other tables like this? Though his discussion was helpful, looks like I need a gear with about 3 more teeth.

Last edited by ScienceguyXJ; September 8th, 2012 at 21:24.
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Old September 8th, 2012, 22:56
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the_weirdo the_weirdo is offline
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Re: Speedo gear replacement to correct for bigger tires and gears

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Originally Posted by ScienceguyXJ View Post
Did this once before on another XJ with 33s and 4.56s that was also off by 10% I believe and called a performance shop and I think they told me I just needed a gear with 10 more teeth. Does that sound right to anyone? Thanks for any insight.
That is wrong. You want to buy one that has a 10% correction, not 10 more teeth.

If you know you are 10% high, pull your current gear and count the teeth. You want to buy one with 10% more teeth.

For example: At 60mph indicated you are reading 66mph on GPS (10% high: 66-60/60). So you want a gear with 10% more teeth which slows down the odometer. If your current gear is 35 teeth, you want to get 38 or 39 teeth which will slow down your reading (35 x 1.10 = 38.5).

Another example if you are reading high: At 60mph indicated you are reading 54mph on GPS (10% low = 60-54/60). So you want a gear with 10% less teeth to speed up the odometer. If your current gear is 35 teeth, you want to get 32 teeth which will speed up your reading (35 x 0.9 = 31.5 = round up to 32 or down to 31).
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Old September 9th, 2012, 00:01
ScienceguyXJ ScienceguyXJ is offline
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Re: Speedo gear replacement to correct for bigger tires and gears

That must have been what the guys at the performance shop told me... 10% more teeth not 10 more teeth.

Just confirmed with GPS that it is reading over by 10% so I'll pull the gear and see how many teeth it's got and go from there. Thanks!
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Old September 9th, 2012, 07:27
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hubs97xj hubs97xj is offline
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Re: Speedo gear replacement to correct for bigger tires and gears

http://www.jeep4x4center.com/jeep-sp...ears/index.htm

There are also some electronic gadgets to correct the speedometer reading, if you can't find a gear that reads accurately.
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Old September 9th, 2012, 09:09
SpeedyVic SpeedyVic is offline
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Re: Speedo gear replacement to correct for bigger tires and gears

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Originally Posted by ScienceguyXJ View Post
That must have been what the guys at the performance shop told me... 10% more teeth not 10 more teeth.

Just confirmed with GPS that it is reading over by 10% so I'll pull the gear and see how many teeth it's got and go from there. Thanks!
Don't make a common mistake. Bigger tires often require a smaller speedo gear. Getting a gear with MORE teeth will increase the discrepancy between your reported (Speedometer) speed and your actual speed. If you are actually going faster than the reported speed by 10%, than you will need a speedo gear that is 10% smaller than the one you have now. You need the gear to complete its revolutions faster, thus reporting to the speedo that the vehicle is going faster.

More teeth equals slower gear. Less teeth equals faster gear.

Edit: I just reread your OP. You said that your speedo is overestimating your speed. In that case, you would need a bigger gear to slow it down. I really hate story problems!

Last edited by SpeedyVic; September 9th, 2012 at 09:15. Reason: My brain hurt!
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Old September 9th, 2012, 09:15
WB9YZU WB9YZU is offline
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Re: Speedo gear replacement to correct for bigger tires and gears

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Originally Posted by ScienceguyXJ View Post
[URL]Unfortunately this doesn't go up to 5.38 gears and doesn't include 37" tires.
These charts are everywhere, Quadratech, Morris Jeep, etc...

They may not list 37 Inch tires, but for the perhaps 1% error you will see by picking a gear which is 1 size lower (For a 36 inch tire instead of a 37inch tire), I would go one tire size smaller on the chart instead of one larger.
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Old September 9th, 2012, 14:20
ScienceguyXJ ScienceguyXJ is offline
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Re: Speedo gear replacement to correct for bigger tires and gears

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Originally Posted by SpeedyVic View Post
Don't make a common mistake. Bigger tires often require a smaller speedo gear. Getting a gear with MORE teeth will increase the discrepancy between your reported (Speedometer) speed and your actual speed. If you are actually going faster than the reported speed by 10%, than you will need a speedo gear that is 10% smaller than the one you have now. You need the gear to complete its revolutions faster, thus reporting to the speedo that the vehicle is going faster.

More teeth equals slower gear. Less teeth equals faster gear.

Edit: I just reread your OP. You said that your speedo is overestimating your speed. In that case, you would need a bigger gear to slow it down. I really hate story problems!
It's all good, this stuff is confusing lol. I do have bigger tires but I also have different gears with 5.38s. My speedometer is 10% over so when I'm going 60mph it's telling me I'm going 66mph. So I need a gear with 10% more teeth to slow it down which corresponds to the tables and equations on the links above.

From the link I posted "Just remember that to speed up the Speedo you need less teeth ( e.g.. bigger tyres ) and to slow it down you need more ( e.g.. going from 3.55 to 4.11 diff ratio )."

I've got a 37 tooth gear so it looks like a 41 tooth gear is the way to go. Thanks guys.
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