The title incorrectly states 4.10 front, 3.9 rear

Yes, thats right.. I think I mistakenly installed a 3.9 front diff into my 4.10 equipt (rear/old front) runner.

In the dirt its not noticable, on pavement... its noticable. Theres resistance in forward/reverse motion. (obviously)

Is it feasable to say screw it, and leave this configuration? This was quite a lot of work for me in my garage with just SUV jackstands. Im in NO hurry to pull it back out. I guess it wont HURT anything, as long as I stay off the pavement in 4WD, (especially at any speed) off road, im sure its fine.

(although it sucks because I know its just not "right")

Tire info: 265/75 16 BFG Mud Terrain.

edit: fixed title

 

A good friend of mine ran a similar setup, 3.91 in the rear and 4.10's in the front for 2 years and had no problems..Now keep in mind he only engaged the 4wd when necessary..His Taco is an Automatic so it wasn't as much of an issue..

 

Yes, my 4Runner has the auto tranny as well.

 

You may not feel it, but your Ujoints and your front axles/CV will have additional stress put on them

 

He's also running 35's and still had no issues..But if run alot would cause undue stress for sure..

 

This will sound odd.. but just let more air out of the front or rear. Change the wheel size a bit, and you will lessen the difference.

Don't ask me about the math to figure it all out though... I am just an idea man.

 

that's what steve (sschaefer3) does since he has 5.29 rear and 5.38 fronts. he lets more air out of the rear since he is only 9 points off. since you're about 19/20 points off, that's a LOT of difference in air pressure

 

Wow... I so want pics of the aftermath.


lllateralus;

You have to understand that what you "feel" on the pavement is very little compared to the stress that's actually going on in the drivetrain. There's a lot between your feelers and the U-joints & CVs to mask the stress.

The math shows that your front end is turning about 5% faster than your rear. That's a LOT. You really need to put the right gears in there.

 

I was just doing the math also, and what you are "feeling" is definately from a very padded standpoint, the additional torque placed on your drivetrain before your tires break (coef of friction based on pretty conservative estimate) is huge... Id avoid 4wd until you can get that fixed correctly

 

Well, I confirmed that it is a different ratio today.

Dirt road, marked all 4 tires at 12:00, 4-hi, drove slowly about
150 feet, (~) front tire marks were down at 6:00, rears were back at 12:00.
(roughly) :pat:

 

Just get bigger tires for the rear until you can get your gears swapped out (that will get you closer, anyways. You may get lucky and match it close enough so that you don't have to change your gears). And as others have said, stay out of 4WD while on pavement.

 

The gears in your old diff should be fine, swap them to the new diff or swap spider gears....

 

Bruce, thats not a bad idea.... although Ive never set up a R/P. I could phisically unbolt/bolt the gears, but as far as precision tools, or proper clearances ect... I dont have the skill or tools.

 

I have a transfer case for sale! If you dont change them your going to be PM'ing me! ALWAYS run the same gears front to rear. Maybe in strict off-road conditions you can get away with it but for 99% of the members here, we all run same gearing front and rear.

James

 

Update: New front 4.10 is installed... 4Runner is perfect again and 4Crawler BL is on the way...


Anyone need a 3.90?

 

I tried selling 3.90's gears for a while when I upgraded to 4.30's. Nobody wants them, although they give way better gas milage.