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Author Topic: Gear ratios  (Read 1348 times)

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Alacran

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Gear ratios
« on: January 18, 2008, 10:10:20 AM »
Sorry if this seems terribly simple but I'm new at this and need all the help I can get. I read the FAQ'S but it left me confused as ever. I have a 94 JY 4cyl with 5 speed. I put 31" BFG AT's on it (actual diameter 30") and I want to recalibrate the speedometer. Am I correct in assuming that I have 4.10 gears? In the Quadratec catalog there is a chart that shows That if I have a 30'' tire and 4.10 gears I need to use a 37 tooth speedometer gear. Does this sound about right to you folks? Thanks for any help!
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black99tj

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Re: Gear ratios
« Reply #1 on: January 18, 2008, 10:28:11 AM »
Here is a great site that will make quick work of your problem: http://www.jeeptech.com/javaAps/Speedo.html
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Offline Jeffy

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Re: Gear ratios
« Reply #2 on: January 18, 2008, 02:25:04 PM »
Quote from: Alacran on January 18, 2008, 10:10:20 AM
Sorry if this seems terribly simple but I'm new at this and need all the help I can get. I read the FAQ'S but it left me confused as ever. I have a 94 JY 4cyl with 5 speed. I put 31" BFG AT's on it (actual diameter 30") and I want to recalibrate the speedometer. Am I correct in assuming that I have 4.10 gears? In the Quadratec catalog there is a chart that shows That if I have a 30'' tire and 4.10 gears I need to use a 37 tooth speedometer gear. Does this sound about right to you folks? Thanks for any help!

You have to know your stock gear ratio.  It should be 4.10/4.11 BUT you might want to check your axles for the ID Tag and see what's in there.  The ID tag is on the differential cover, held on my two bolts at the 2 o'clock position usually.
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Alacran

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Re: Gear ratios
« Reply #3 on: January 18, 2008, 05:01:54 PM »
Thanks for the help guys! This forum is loaded with information.
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Offline jfrabat

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Re: Gear ratios
« Reply #4 on: January 21, 2008, 01:13:31 PM »
IF (BIG IF) you have 4.10 gears (you probably do, but check anyway), then yes, it sounds about right.  I have a 38 tooth speedo gear on 31's, and the speed on the dash is almost always on par with the speed on my GPS (sometimes the difference is 1 mph when I am doing 65~70 mph, but that is negliagable).
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Offline Bounty Hunter

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Re: Gear ratios
« Reply #5 on: January 24, 2008, 11:42:39 AM »
Quote from: Alacran on January 18, 2008, 10:10:20 AM
Sorry if this seems terribly simple but I'm new at this and need all the help I can get. I read the FAQ'S but it left me confused as ever. I have a 94 JY 4cyl with 5 speed. I put 31" BFG AT's on it (actual diameter 30") and I want to recalibrate the speedometer. Am I correct in assuming that I have 4.10 gears? In the Quadratec catalog there is a chart that shows That if I have a 30'' tire and 4.10 gears I need to use a 37 tooth speedometer gear. Does this sound about right to you folks? Thanks for any help!
I would go by advertised height and not actual height, stick with 31" in the charts as I believe that's how they are set up.  Yes, I know there are a lot of variances between tire manufacturers but I think 31" will get you the closest.
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Offline Jeffy

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Re: Gear ratios
« Reply #6 on: January 24, 2008, 12:21:25 PM »
Quote from: Bounty Hunter on January 24, 2008, 11:42:39 AM
I would go by advertised height and not actual height, stick with 31" in the charts as I believe that's how they are set up.  Yes, I know there are a lot of variances between tire manufacturers but I think 31" will get you the closest.

Yes, Terry uses advertised height not actual height in his calculations, which is why it's not perfect.
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Alacran

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Re: Gear ratios
« Reply #7 on: January 25, 2008, 11:50:53 AM »
OK.... Well I went ahead and ordered the 37 tooth. I'll let you know how it turns out.
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