FT4WD / Tire Size

[n1265]

I thought I read somewhere that you could not run different size tires on a full time 4wd, But I am not sure.

anybody know ? I have a spare that is slightly larger than the 4 I am currently running...

[greg rider]

Well,,,you're not suppose to. If you don't have locking diffs and you have a 203 transfer case in "HI" not "HI-LOC" you can do it for a little while. I would only do that if I had a flat with an odd spare.

[2x1972]

You can run them, as long as 4x4 or diffs are not locked in. It is my understanding that there is a differential in the transfer case of the full time trucks that allows the front and rear to spin at different rates, allowing the truck to go around corners without bouncing and breaking parts. This same system will allow you to run a tire that will spin your axle shaft at a slightly different rate.

However I would not recommend driving your truck like this for any longer then you absolutely have to. The spider gears in your diff are designed to let the axles turn at different rates when cornering, but stay put when going straight down the road. Making them work constantly by running different tire sizes will cause them to heat up while you drive, leading to premature failure.

[n1265]

Ok, What if you run the same size on an axle, but different size from front to rear ?

[2x1972]

Same deal since there is something in the T-case working to keep them turning at the same rate...

Plus it defeats the purpose of owning a 4x4 because as soon as you lock it in, the two axles will be fighting each other with different rates... think of it like this: if you had 4.10 gears in the front, and 3.93's in the rear, they would fight because the driveshafts are locked in at one spin speed, but the two axles require a different amount if driveshaft input to turn the tires the same distance. when you run different size tires front and rear you are making the driveshafts turn a different amount for the tires to travel the same distance.

So on a 2wd truck this causes no problem whatsoever, but on a 4x4 it wont work. Again it is possible to do in an emergency situation, just not for extended periods of driving.

[n1265]

Many Thanks for the info ...