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Discussion Starter #1
Does anyone know the correct speedometer pinion gear tooth count for the 3.92 ratio axle with the 31x10.50R15? I just bought 31s but the truck came with 235s. I'm getting conflicting info on whether I need to go to a ~33 tooth gear or a 38 tooth gear. It's a 97, 3.9L, 4x4, 42RE, 3.92.
 

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Going from the stock tires to the 31 inch tires will change your circumference by about 7.3 % (90.7 inches to 97.3 inches). That's not a lot of difference - 60 mph indicated would be actually 64.4 mph.

Looking at the gears available (28 tooth, 33 tooth, 38 tooth, etc) each gear represents a 15% to 18% difference from the next size, or at least double the change you need. So instead of reading 4 mph fast, your speedometer would be reading 4 - 5 mph slow. I haven't actually done this on my Dakota, I'm just crunching the numbers available at different web sites. I'd suggest just using the stock gear, and keeping my speedometer slightly under the desired speed (or using a GPS to calculate your speed).
 

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Going from the stock tires to the 31 inch tires will change your circumference by about 7.3 % (90.7 inches to 97.3 inches). That's not a lot of difference - 60 mph indicated would be actually 64.4 mph.

Looking at the gears available (28 tooth, 33 tooth, 38 tooth, etc) each gear represents a 15% to 18% difference from the next size, or at least double the change you need. So instead of reading 4 mph fast, your speedometer would be reading 4 - 5 mph slow. I haven't actually done this on my Dakota, I'm just crunching the numbers available at different web sites. I'd suggest just using the stock gear, and keeping my speedometer slightly under the desired speed (or using a GPS to calculate your speed).
I've seen the individual counts ( 28, 29, 30, 31, 32 etc.) available at places like LA Speedometer Gear (disclaimer: This store is in my neck of the woods, I've met Kevin, he's a cool guy. He isn't the ONLY one listing them, though! Feel free to Google someone else.) as seen here: http://www.laspeedometergear.com/dodge jeep speedometer gear.htm

And *ahem* If anything, have the speedo indicate a bit HIGH. That way you can truthfully say "It said I was doing 65 in a 60" and the LEO will go "Well, you were doing 64. Keep an eye on it." *grins*

RwP
 

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Thanks, Ralph. OP mentioned 33 tooth and 38 tooth, and that matched what I saw as available from dealers. I wasn't aware there were other options.

In that case, take the original gear tooth count and multiply it by 1.073, and select the gear count closest to that. For example, if your original gear had 30 teeth, 30 x 1.073 = 32.19; select the 32 tooth gear. (1.073 is the ratio between the diameters of the 31x10.5R15 tire and the 235/75R15 tire)
 

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Discussion Starter #5
I've tried the online calculators before and still didn't get a good result. This is actually my second 97 Dakota. The first one had the NV1500 and came with the 31" tires. I dropped down to the 29" tires and I believe I pulled out a 36 tooth gear and replaced it with a 34 gear based on the calculators. I then got a speeding ticket and took the truck to a speedometer calibration shop to find out I was like 9 MPH off.

I was hoping to find someone who knows what their truck had to start with so I could get close. I'd rather not have to do mental math to know if I'm going faster or slower than the indicated speed. I'd rather it be as close as possible to the correct number and I can decide if I want to speed and by how much.
 

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Well, here's an easy method if you have a flat highway and a friend.

Find about 3 miles of flat land with mile markers.

Do an indicated 60MPH.

When you get to the first one, have your friend start a stop watch; at the end of the run (1 mile will do, but 3 or so gives some averaging to help offset the variance in speed that normally happens) stop the watch.

See how long it was in minutes; if it's 3 minutes for 3 miles. you're right on. If it's 2 minutes 45 seconds, or 2.75 minutes, then you need a gear that's 2.75/3 the number of teeth you currently have. If it's 3 minutes 15 seconds, or 3.25, then it's 3.25/3 teeth. (More teeth, slower speedometer. Fewer teeth, faster speedometer.)

Then pull your current gear long enough to read the number on it. Do the math from that, and presto, you have your right gear.

I'd opt for the speedo reading a bit HIGH myself.

There's always a chance your speedo was reading low already; with the gears you used, it was reading even MORE low.

RwP
 

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Discussion Starter #8
I used my GPS speedometer on my data logger. 60 mph indicated showed as 62/63 mph on the GPS with the red, 36-tooth. I swapped for the green, 34-tooth gear and now 60 mph indicated is 58/59 mph. What I really need is a 35-tooth, but I'm happy with the 34-tooth.
 

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If I remember correctly, our trucks use the long gear ...

https://www.ebay.com/itm/New-Dodge-...peedometer-Gear-Jeep-New-Process/172854532528

There's also a short shaft drive gear, but I don't think that's the right one for our transmissions. Do check, though; I don't have a gear in my hand to look at.

I will add that some local to me folks have them, too: https://transmissioncenter.net/product-tag/speedometer-gears/

Scroll down to both the short and long shaft Dodge gears, and pick the 35 tooth variant.

RwP
 

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Discussion Starter #12
It's the short gear. And my speedometer is reading high. When it reads 60 mph I am actually traveling 58 mph. If I went with more teeth it would be closer to reading 60 mph when I am traveling 60 mph.

2018-06-07 20.06.43.jpg
 
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