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Discussion Starter · #1 ·
At about 70 mph on the Interstate I'm running about 3000-3100 rpms. I have 3.73s and tire size is 285/75/16. Is that to high or is that about normal and could it hurt anything?
 

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I have 33s with 3.73 gears and I run about 2400 at 70 mph.
 

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I can't tell you the exactly way to do it but you lift up the rear and spin the tire counting the rotations as compared to the driveshaft. Maybe try googling that or someone else will chime in.
 

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Just lift up one tire in the rear. Put tape on the drive shaft and tape on the tire so you can more easily count and see when you got back to the beginning. Spin the tire around twice and count the number of times the driveshaft turn.

You spin it twice because of the way the spider gears work. With on tire sitting still and the other moving the driveshaft moves half as fast as it would with both tires moving.

I'm pretty sure that's right... Haha
 

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At about 70 mph on the Interstate I'm running about 3000-3100 rpms. I have 3.73s and tire size is 285/75/16. Is that to high or is that about normal and could it hurt anything?

It's high for a true 70 mph but 3000 rpm isn't going to harm the engine. I've run the same rpm at ~80 mph on my daily commute for the last seven years. According to grimmjeeper you would be at 2672 rpm at 70 mph with a 32rh,3.73,285/75/16 combo. 4.10s would put you at 2937 rpm.
 

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if you put a 17" tire you might get the rpms to drop a little.maybe the po had bigger tires if you wanna check you speedometer get a garmen gps unit.most tell your speed when your rolling.mine came stock with 16" and i found a set of stock 17" take-offs.now when my speedometer says 60mph my gps says 66mph..its worth a try
 

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Do you have the 3 speed or 4 speed auto? I have the 3 speed auto with 3.73 gears and I was pretty close to 3000rpm at 70mph when I was on 31s. (Quite a bit lower on 33s now.) Is it possible your speedo gear isn't matched to your tire size? You could download a speedometer app for your phone to check.
 

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if you put a 17" tire you might get the rpms to drop a little.maybe the po had bigger tires if you wanna check you speedometer get a garmen gps unit.most tell your speed when your rolling.mine came stock with 16" and i found a set of stock 17" take-offs.now when my speedometer says 60mph my gps says 66mph..its worth a try
Do you mean a 17" wheel? If so, it's only the tire diameter that affects engine rpms, not the wheel diameter.
 

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Discussion Starter · #15 ·
I know my speedometer was off 7 mph when I had 31s. Now that I got 33s I'm sure it's gotten worse. So when I wanna go 70 I usually get my speedometer up a lil over 60
 

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I know my speedometer was off 7 mph when I had 31s. Now that I got 33s I'm sure it's gotten worse. So when I wanna go 70 I usually get my speedometer up a lil over 60
Sounds like it's time to invest in a new speedo gear. $40 and really only a 5 minute job even for a novice wrencher like me.

Also, if it had 31s (or even smaller) on it when you got it, there is pretty much zero chance it was regeared so if your build sheet says you have 3.73 gearing that's almost certainly what you still have.
 

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Determine your current gearing. Once you have that do a search on the forum for speedo gear. There is a chart that will tell you what speedo gear you need with gear ratio and tire size.

You can get a new gear off E-bay for around $25.00 quite often. I just replaced mine to correct speedometer after upgrading to 4.88 gears. Literally a 5 minute job.

To address you RPMs? Your RPM is not too high for the 4.0. I think you are geared lower than 3.73. Jack TJ up and count the driveshaft revolutions vs the tire rotation. The tape method referenced above will get you the info needed. You tube has some good videos showing you how to do this.

Hope this helps.

Happy Jeepin'
 
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