Upgrade to 3.73 or 4.10 gears with 31" tires?

[Pat Goode]

Hi,
Is anyone running on 31" tyres with a 3.73 or 4.10 Diff ratio.
I'm trying to make my mind up as to which way to go. 31" tyres are plenty big enough for the type of off roading I do. In the uk we need to keep the entire tyre within the wheel arch.
It would be easier to re-gear to 3.73 as replacement front axles are readily available off XJ's and the 3.73 is within the carrier break off the, As fitted Dana44 Trac-lok rear diff
I'm fine with the present 3.07 ratio on the tarmac, But off-road in low range the gearing is much to high.
Any observations would be greatly appreciated.
Difficult to find any advice as in the States you all seem to favour 33" or 35" Tires

 

[Chris]

Picking the ideal gear ratio depends on a number of variables:

1) Tire Size
2) Manual or Automatic Transmission
3) Engine (4 cylinder or 6 cylinder)
4) What year is your TJ? (this is to determine which automatic or manual transmission you have)

Before we can give you an ideal gear ratio, you have to answer those questions.

 

[Pat Goode]

Can I alter the heading as I meant to put 31" tyres

 

[Chris]

Can I alter the heading as I meant to put 31" tyres

I changed it to 31" for you.

 

[Alex01]

Like Chris said more info is needed for an accurate recommendation. That said I would first ask yourself what size tire do you see your jeep running in 3 - 5 years.

I have a 5 speed transmission and ran 4.10 gearing with 33s inch tires. It was fine but 4.56 would have been better. I now have 35 inch tires and run 4.88 gearing which for my mixed use of trails and highway is the perfect happy medium. If I did more trails then highway I would have choose 5.13.

I would assume that 4.10 would be a better choice then 3.73 with 31 inch tires.

 

[bobthetj03]

I ran 31's with a 4.0L and a 5 speed with 3.73 gears. It was acceptable.

 

[Jerry Bransford]

I would assume that 4.10 would be a better choice then 3.73 with 31 inch tires.

The factory agrees, 4.10 is what they install into Rubicons which come standard with 31s.

 

[StG58]

The factory agrees, 4.10 is what they install into Rubicons which come standard with 31s.

 

[theRealCrazy]

I have 31s on 3.73 /4.0 auto , I see no issues, however i bought it that way so i can't compare.

 

[MutantNinjaTJ]

I would go with the 4.10’s. Mine has 31’s and is presently running 3.73 with a 6 speed. It’s OK, but could be better. Going to 33’s and 4.56’s first week in October. Can’t wait!

 

[Pat Goode]

My TJ sport is a 1997 4L 6cyl 5 speed manual RHD,Export model with the Dana 44 Trak Loc rear axle.

Highway performance and fuel economy is not my greatest concern. (LPG Conversion) Its a hobby vehicle, not a daily driver. I will mostly be using it for off road use and unfortunatly we don't have the wide trails in the UK that you have in the States.
I do a lot or what we call over here, RTV's. (Road Taxed Vehicle Trials) Which involves navigating through an off road course made up of sections, These are quite narrow and tight, hence the need for smaller tyres close into the body. The main advantage is having good low range gearing.
Obvously Re-Gearing is a compromise, as like most of us I don't have an unlimited budget. 3:73 would be easier and cheaper to obtain than 4:10. Unfortunatly Wrangler are quite rare in the UK and donor parts scarce.

 

[Jerry Bransford]

I have 31s on 3.73 /4.0 auto , I see no issues, however i bought it that way so i can't compare.

That would work well since you have the older 3-speed automatic which works great with your combination. If you had a 2003 or newer which has the 4-speed automatic, you'd hate it since its overdrive ratio drops the highway rpms way lower than what works well which causes the engine to lug.

 

[CMTJ]

I run 31's with an auto trans, 2006 X model. I have zero issues even going over a bridge twice a day for work. I keep up with traffic and run about 70mph on the bridge at about 2300 rpm's or so. Maybe a little higher in rpm's not sure. All with stock 3.73 gears.

 

[WDE]

I have 31s on a 2000 4.0 5-speed with 3.73 gears. It is decent but could be better in 5th gear where it is quite sluggish.

 

[Pat Goode]

Ah that’s interesting that even with 3.73 it’s sluggish in 5th.
Any observations on what it’s like in low range?

 

[Alex01]

My TJ sport is a 1997 4L 6cyl 5 speed manual RHD,Export model with the Dana 44 Trak Loc rear axle.

Highway performance and fuel economy is not my greatest concern.

4.10 hands down for a daily driver. If high speeds really mean nothing then 4.56 and you'll be ready if you move to 33s one day and will have a great crawl ratio.

Since you have a 1997 I will assume you have an AX15 so your 1st gear ratio is 3.83 the Rubicon didn't exist yet so you have a NP231 with a 2.72 ratio. Your variable is gear ratio of 3.73, 4.10, 4.56 (3.07)

crawl ratio = transmission ratio x transfer case ratio x axle gear ratio

3.07 = 31.98 (baseline, current speed in 4low)
3.73 = 38.86 (22% slower)
4.10 = 42.71 (34% slower)
4.56 = 47.50 (49% slower)

 

[S.McArthur]

I have 3:73/AT/32" rubber, I do well enough on the highway but would prefer a better gear set. I have been eyeballing the 4:56 when I finally get a new rear axle set up and 33" tires.

 

[Pat Goode]

Alex
Thanks thats great info! I definatly won't be tempted into a 3:73. 4:10 is a possiblity, but 4:56 looks ideal for use in the UK.
I found this link to a gear ratio calculator which is superb. Might have been from a link on this site, but worth re-posting


http://www.grimmjeeper.com/gears.html

 

[AndyG]

The rule is you have to do exactly what Jerry approves.

As long as Chris is good with it.

On a serious note I commend any one for regearing their Jeep.

If you don't know how to do it I commend you for hiring someone.

 

[Alex01]

The rule is you have to do exactly what Jerry approves.

As long as Chris is good with it.

On a serious note I commend any one for regearing their Jeep.

If you don't know how to do it I commend you for hiring someone.

@Jerry Bransford always approves of 1 size up in gearing if you don't care about highway speeds