31x10.50x15 and re-gear?

Jeepin'WNC · **Joined:** Mon Nov 15, 2010 1:48 am **Posts:** 191

Well, the subject pretty much says it. I recently mounted some 31x10.50x15 Goodyear Duratracs load range C tires on 15x7 steel rims. I have 3.73:1 gearing. Man, my MPG's dropped significantly, and the Jeep seems to struggle a little to comfortably maintain a decent hwy speed. I thought when I bought the tires that they were comparable in size to 245/75/16, but now I find that they are closer to 265/75/16. Will it be worth the expense in the long run to re-gear to 4.10?
I'd love to be able to afford to tear into both diffs and do a re-gear, ARB in the rear and Detroit tru-trac in the front, but man! that's some serious cash!!

tommudd · **Posted:** Wed Dec 19, 2012 5:49 pm

Most 31X10.50s are in the 30.5 -30.8 range
245-75-16s are 30.7 -30.8 or so
so about the same
But yes even moving to that size you notice the power drop, add in steel wheels etc and you'd be wise to regear
You wouldn't believe the difference in how it would go then

Jeepin'WNC · **Joined:** Mon Nov 15, 2010 1:48 am **Posts:** 191

Yeah, I'm thinking I may have to re-gear in the near(er than I would like) future...The Libby is gettin' a little heavy...

spitfire36o · **Posted:** Thu Dec 20, 2012 11:56 am

Calculate your revolutions per mile and have the dealer re-calibrate the speed sensor. Your shift points are set to specific speeds. With the larger tire, you are shifting at a higher speed than you should. The speedometer is reading 4 - 4.5% low. I put on 30.75" Treadwright warden AT's and it was painful driving until i went to the dealer. They charged $55 and i changed it to 680 revs per mile. My speedo is dead on with GPS now.

Your tire is listed at a 30.5" diameter
Which means the circumference is 95.8186"
There are 5280' in a mile x 12 is 63,360" per mile
63,360/95.8186
661.25 revs plus 3.5% for squish
684.39 revolutions per mile

Tire rack specs have it listed at 685 revs - so they check out!

If you re-gear, I believe you will have to do this all over again. So if you aren't going to re-gear for a while, i would spend the money to re-calibrate twice.

tommudd · **Posted:** Thu Dec 20, 2012 12:52 pm

Still will be a dog until he regears
trust me been down that road before :wink:

tjkj2002 · **Posted:** Thu Dec 20, 2012 1:25 pm

spitfire36o wrote:

If you re-gear, I believe you will have to do this all over again. So if you aren't going to re-gear for a while, i would spend the money to re-calibrate twice.

No need to have the speedo recalibrated after gear change unless you have a '06 or '07 KJ.

spitfire36o · **Posted:** Thu Dec 20, 2012 4:08 pm

tjkj2002 wrote:

No need to have the speedo recalibrated after gear change unless you have a '06 or '07 KJ.

So then go for the re calibration ASAP. At least you will have the proper shift points back.

tommud wrote:

Still will be a dog until he regears
trust me been down that road before

I have a CRD, so i guess the torque down low makes a big difference when you go with the tires and stay with the 3.73s

tommudd · **Posted:** Thu Dec 20, 2012 6:34 pm

Small amount but you'd still see a huge difference regearing
No matter what engine you are using they have a sweet spot where that particular vehicle runs good at hiway speed ( RPM/Speed) If you say go to a larger tire and drop that RPM at say 65 to 4-500 RPM it will bog down and run harder

spitfire36o · **Posted:** Thu Dec 20, 2012 7:19 pm

tommudd wrote:

Small amount but you'd still see a huge difference regearing
No matter what engine you are using they have a sweet spot where that particular vehicle runs good at hiway speed ( RPM/Speed) If you say go to a larger tire and drop that RPM at say 65 to 4-500 RPM it will bog down and run harder

3.7L Gas
Torque (SAE net): 235 lb.-ft. @ 4000 rpm

2.8L Diesel
Torque (SAE net): 295 lb.-ft. @ 1800 rpm

Stock Sport Tire/3.73 rear: 65 MPH = 1,946 RPM
Stock Limited Tire/3.73 rear: 65 MPH = 1,907 RPM
30.5" Tire/3.73 rear: 65 MPH = 1,870 RPM
30.5" Tire/4.10 rear: 65 MPH = 2,055 RPM

I can understand it looking at these figures on paper why a gasser would really benefit from the regear with the larger tires because you don't get into the power until the rpms are up. But can anyone with a CRD with tires and a re-gear shed some light on this. I would think your city mileage would be better but not much on highway. Would the gains on a CRD be worth the cost.

tjkj2002 · **Posted:** Thu Dec 20, 2012 9:27 pm

spitfire36o wrote:

tommudd wrote:

Small amount but you'd still see a huge difference regearing
No matter what engine you are using they have a sweet spot where that particular vehicle runs good at hiway speed ( RPM/Speed) If you say go to a larger tire and drop that RPM at say 65 to 4-500 RPM it will bog down and run harder

3.7L Gas
Torque (SAE net): 235 lb.-ft. @ 4000 rpm

2.8L Diesel
Torque (SAE net): 295 lb.-ft. @ 1800 rpm

Stock Sport Tire/3.73 rear: 65 MPH = 1,946 RPM
Stock Limited Tire/3.73 rear: 65 MPH = 1,907 RPM
30.5" Tire/3.73 rear: 65 MPH = 1,870 RPM
30.5" Tire/4.10 rear: 65 MPH = 2,055 RPM

I can understand it looking at these figures on paper why a gasser would really benefit from the regear with the larger tires because you don't get into the power until the rpms are up. But can anyone with a CRD with tires and a re-gear shed some light on this. I would think your city mileage would be better but not much on highway. Would the gains on a CRD be worth the cost.

I know JLRockies went to 4.10's when he still had his CRD and said it was a huge improvement over the 3.73's.

tommudd · **Posted:** Thu Dec 20, 2012 11:15 pm

Been a couple of others did as well plus I think the one guy went with a steel front diff / 33s and 4.56s with his CRD
I'm thinking it would even be better with a CRD than a gasser

blackwidowkj06 · **Posted:** Fri Dec 21, 2012 10:01 pm

My KJ has 3.73 gears, runs on 31x10.5x15LT BFG AT T/As mounted on black steel Cragars. It gets around pretty good. Maybe one day when the funds are there I will regear.

Regearing costs so much, you have to decide if the costs is worth the reward in the long run. Unless you can do the labor part yourself. If you daily drive your KJ a lot, IMO it would be worth the cost. As much as I drive mine, I wish it was regeared at times. It would have paid for the costs a long time ago.

31x10.50x15 and re-gear?

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