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I have a Ford F150 4x4. The front 2 tires treads measure 12/32, one rear is 6/32 and the other is 10/32. The dealership told me I should replace the rear 2 tires because the big difference in tread depth will cause the tires to spin differently and could damage my 4 wheel drive system.

Is this really a concern or are they just trying to sell me new tires?

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3 Answers

This is something I've commonly heard said, usually by tire salesmen, but I do not buy. The difference in speed of the rotating tires between 6/32" is very small and likely less than many other factors such as alignment and tire inflation differences. The differentials are meant to handle small differences between the different tires rotation speeds and should be able to cope with this.

All this assumes that you are using the same size tires. Never mix tire sizes on a 4WD vehicle.

However, I'd recommend that you call Ford if they provide some sort of "answers line" and verify with them that there isn't anything unique in the design of the system that couldn't cope with this difference in tread depth on otherwise identical tires.

As an aside: We were once traveling through Montana and had a tire die. We limped into town and found a place that could replace the tire, but they were pushing for us to get all 4 matched tires, because we were driving an AWD car (Audi Quattro full-time AWD). I'm kind of particular about my tires and they didn't have what I would have wanted on it, so I just had them replace the one, so we could get home and deal with getting the right tire ordered in then. I was able to get them to replace just the one tire.

I later called the Audi Customer Service Hotline and asked about this. The person I spoke to said that as long as they were the same size tire, it was absolutely not a problem. She re-iterated that the tires must be the same size, but differences in the wear are fine. She obviously did not look this up or have to ask anyone, so it seems like a question they get a lot.

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The only case I know of where it would make a difference is if you have a locking centre differential and drive on hard surfaces with it engaged (which you shouldn't do anyway) –  Nick C Jun 1 '11 at 8:36
    
The primary case I'm worried about is that Ford HQ says it's an issue, whether it really is or not, and invalidates the warranty because of it or something crazy like that. You know, one of those sneaky lawyer tricks. :-) –  Sean Reifschneider Jun 1 '11 at 21:04
    
You may not buy the line from the tire salesman, but you may end up buying a new differential if you ignore what the vehicle's manufacturer has to say for your exact model. –  qes Jun 7 '11 at 23:36
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@qes: I believe that's what I said. –  Sean Reifschneider Jun 11 '11 at 3:02
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Different outer tire diameters creates a different rate of rotation for each wheel. Depending on the design of the differential, maintaining higher speeds for a lengths of time with great enough differences in rotational speeds can create too much heat, which I imagine leads to break down of the lubrication and damage to the differential.

It would be wise to follow the manufacturers guidelines.

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There is no problem with driving different tyre tread depth. Imagine that you go on slight turning angle. Wheels have different rotation speeds anyway even on high speeds. So differentials do all the job to make drive smooth. There is no big deal driving few milimiters smaller wheel on one side since it is not more than 10mm or something about 1/25 inch.

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