When my truck had about 17k on it I took some road shrapnel in one tire. Huge gash that couldn't be fixed. Bought a replacement and off I went. Not at about 23k I haven't rotated the tires since because the new tire has so much more tread I was afraid it would induce extra wear on my rear diff. Now I'm wondering if I'm worried for nothing and should just rotate away. Any opinions on this? The new tire has quite a bit more tread and therefor more overall height than the other 3.

 

Maybe just rotate the 3 and see how she goes. IMO you're thinking too much. It's a truck, I'm not sure you'd notice a difference.

 

I'm going on this theory. My tires are the optional 20" ones. My spare is an 18". Almost the same height, but not quite. My manual says that I should always put the spare on the front as different height tires spin at different speeds. If you have them spinning at different speeds, the rear anti lock diff clutch is always slipping even when driving in a straight line making it cook faster. Then again, your probably right. It probably won't make any difference and I should just rotate....

 

What is the height difference between the spare tire and the tires on the truck? Probably more then the difference between the new tire and the old tires?

 

Just rotate the tires and have a beer. You're over-thinking the problem.

 

Its the point of a differential to allow tires to spin at different speeds anyway? So a different size tire shouldnt make a difference. I think..

But then again I'm no expert..

 

Right, but in a straight line the clutch isn't slipping. Only when cornering. With two different height tires, the clutch is always slipping. Hence the premature wear. But yes, it's probably so small of a difference that I shouldn't worry. Going to add a rotation to the to do list......

 

Karl.....you have wayyy too much time on your hands. God forbid your brakes start to squeal. As the OP said, your over thinking the situation.

 

What is the actual thread depth measurement of all the tires? I would think the tires should be close enough not to worry but if your old ones are almost to the bars and the new one is, well, like new yet then I would worry a bit. I have heard clutch type LSD's boiling fluid before because they got so hot from different size tires (same tires just one wore down and other not) running on the highway.

 

If you have the stock tires with a non aggressive tread I don't see how your going to have a problem. When I rotate tires I always put the tire with the most tread from the front on the right rear. The right rear is the first to spin and wears tires down quicker than the other 3.

 

Well, some quick math (someone check my work)...

Assuming a 265-75 on a 20" wheel, diameter would be 35.65". If we use that for the newer tire, and round it down to 35" for the worn tire:

35" = 110" per revolution = 576 rev per mile

35.65 = 112" per rev = 565.7 rev per mile

AT 60 mph that means your clutches are dragging at about 10 RPM. Doesn't seem too bad, but I guess it depends on the preload. Also, my numbers are assuming a 0.325" difference in tread height, which is probably higher than what it actually is.

Many people here would say that the Ford LSD's are worthless anyway, so wearing the clutches out might not affect anything anyway

 

you are right to be concerned, running tires of different diameters in the rear will kill your LS, i wouldn't do it. as far as a solution goes, it seems that people have already posted the best options for you.

i was just thinking, this problem will never go away as the difference in tread depth with be maintained through out the life of your old tires. the only solid solution would be to purchase another new tire.

good luck to you.

 

Thanks for the thoughts guys. At least you all are split 50/50 on this....LOL.... So in response, I have tomorrow off and I'll either rotate them or I won't. LOL. I just went out there and measured the two tires that will go to the rear. There is 1/8 inch difference. Guess I won't sweat it.

 

^^^ this ^^^

 

That's friggen hysterical. Just rotated, hit the showers, and cracked a cold one.... How'd ya know???