Recommended Gearing 37s on a CRD - JKowners.com : Jeep Wrangler JK Forum
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post #1 of 3 Old 11-18-2016, 01:23 AM Thread Starter
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Question Recommended Gearing 37s on a CRD

Hi,

I have a 2007 JKU 2.8 CRD, currently wheeling 35" tyres on 4.1 diff gearing.

Am upgrading the axles to dynatrac front and rear (PR44/PR60) and moving to 37" tyres (why not eh?). (37x12.5x17 KM2).

I need some help on the best gear ratio, I'm thinking of either 4.56 or 4.88, shorter than that and the top speed will start to drop to 65 mph.

Does anyone have experience of the CRD on 37s?

Daks
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post #2 of 3 Old 11-18-2016, 08:16 AM
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I only wish I had a CRD!

Here is my expereince with JKs:

2007 3.8L 37s and 5.38 gears

2014 3.6L 37s and 5.13

2014 3.6L 35s and 4.88

4.10 to 4.56 is not much of a change and if you were disappointed with the small performance gain, it's costly to change to 4.88s. Also consider rolling resistance with the 37s. I would highly recommend the 4.88s. Your low end torque will make up for a lot of gearing but it is so nice not to use very much throttle on and off the trail. I ran my 37s for about 6 months on 4.10 gears before finally getting around to installing 5.38s. Even with all the power in the world the JK is still like a freakin plow down the highway. The drag coefficient is equal to a brick wall.

Hope this helps.


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post #3 of 3 Old 11-19-2016, 05:31 AM Thread Starter
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Quote:
Originally Posted by Rubi-8 View Post
I only wish I had a CRD!

Here is my expereince with JKs:

2007 3.8L 37s and 5.38 gears

2014 3.6L 37s and 5.13

2014 3.6L 35s and 4.88

4.10 to 4.56 is not much of a change and if you were disappointed with the small performance gain, it's costly to change to 4.88s. Also consider rolling resistance with the 37s. I would highly recommend the 4.88s. Your low end torque will make up for a lot of gearing but it is so nice not to use very much throttle on and off the trail. I ran my 37s for about 6 months on 4.10 gears before finally getting around to installing 5.38s. Even with all the power in the world the JK is still like a freakin plow down the highway. The drag coefficient is equal to a brick wall.

Hope this helps.
Thank you for your post, yeah its a brick wall alright.

I've had the CRD chipped and have put a sports exhaust on it, so am getting around 600 NM of torque at 1600 RPM, which is pretty much unstoppable. So much so I need to upgrade the axles.

I think it has a similar torque to the hemi v8, just doesn't rev beyond 4000 RPM which is the problem for short gears, although it is great for being 25 miles per gallon btw

You might be right about 4.88.

Ive been through the maths and with 4.88 at 70 mph, with a 37 inch tyre, assuming it drops to 36 1/4, then I would have 2100 RPM in 5th gear.

At 5.13 = 2250 which would be a bit revvy for the diesel.
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