Gear on a YJ with 31"

[jeepfan555]

Well like I said before I am getting 31" Pro comp Xterrains, the only thing that I am concerned about now is if and how the gearing is going to change and also the speedometer. How much will it change and will I loose a lot of power on the road.

 

[Bounty__Hunter]

What engine and transmission do you have now? What axle gear ratio?

31" isn't too bad, your speedo will read about 3-4 mph slow.

 

[Snitty]

it will be 3-4 slow at about 55 mph... but onlt about 2 slow at 35,the faster you go, the more off it will be

 

[Bounty__Hunter]

Snitty, are you sure? I thought the tire change affected the speedo by the same percentage that the tire height changed, across the board. I figured the change was constant despite the speed.

 

[jay79cj7]

The percent your speed is off is constant, but the absolute value of the error is proportional to your speed. If you are off 2 MPH at 30, you will be off 4 ad 60, etc.

 

[88Wrangles]

The formula to figure it out is: Actual MPH= New Tire Diameter/Old Tire Diameter x Indicated MPH

 

[88Wrangles]

Just make sure you use the actual tire size and not just what it says on them!

 

[TwistedCU]

I would get them on there before you worry too much about gearing. See how they work out and go from there!

Handy tool for actual speed with larger tires:

http://www.offroadtoystore.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=GearCalculators

This chart shows, for example, at 55mph on the speedo, you will actually be doing 61.56mph if your stock tire size was 215/75/15 (27.696850393700785 actual inches) and your new tires are 31".

 

[hi_c]

Yes, it's a percentage, not a fixed value. I have 4.10 axle ratio and when I went to 31's I found it to be about a 10% difference. Here's how I came to that conclusion. Get on the interstate, or any highway with mile markers. When you pass the mile marker, reset your trip-meter. Then watch and see when you pass the next mile marker. I found that I only registered 9/10's of a mile while completing 1 mile. Therefore, we can conclude that your speedometer and odometer will both register a 10% difference. So if your speed says you're doing 60, you're really doing about 66. So watch your speed around the black&white.

C

ps. to correct this, either re-gear your axles, or get a new speedometer gear put in your t-case to make up the discrepancy.

 

[TwistedCU]

Pennsoil Synchromesh

A handheld GPS makes it easy too!

 

[hi_c]

Yeah, there's that.

 

[Bounty__Hunter]

It's nice having a speedo on the GPS, those things are pretty accurate to check the jeep's speedo.