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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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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. – quentin-starin Jun 7 '11 at 23:36
@qes: I believe that's what I said. – Sean Reifschneider Jun 11 '11 at 3:02

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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The differential isn't the problem so much as the front-rear transfer case. – Juann Strauss Jul 4 '14 at 8:49

It matters, but how much it matters varies from vehicle to vehicle. TireRack has an article about this topic -> Matching Tires on Four-Wheel Drive and All-Wheel Drive Vehicles

Here is the Manufacture Specific info from the end of the article:

Here are recommendations from some of the manufactures that Tire Rack currently serves for matching the tires used on their four-wheel drive and all-wheel drive vehicles. Additional recommendations from other Original Equipment Vehicle Manufacturers is pending.

  • Audi: As published in their vehicle owner's manual, "rolling radius of all 4 tires must remain the same" or within 4/32-inch of each other in remaining tread depth.
  • Porsche Cayenne: within 30% of the other tire on the same axle's remaining treadwear.
  • Subaru: Within 1/4-inch of tire circumference or about 2/32-inch of each other in remaining tread depth.

Unfortunately they don't have any info specific to Ford so you should definitely call Ford & get their take or check your owners manual and see if it is in there (like it is for Audi).

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Look, I agree with the tyre rack article to a certain extent. But let's all use a little common sense. Using their example (as I do) in rotating my full size spare on my Wrangler. Even when I swap out at every tyre rotation the act of not using 1 tyre for that set of 5000 miles creates a tyre circumference difference of about 1/8" of an inch. So by there example, I could not ever rotate that tyre back on to my driveline b/c of the 4 less rotations per mile that larger tyre would roll. Not to mention that all roads/ highways are angled or crowned to allow water to be diverted off so that creates a difference in rotation numbers as well. I think the moral is keep it close, if your pushing anything more than an 1/8 or more I wouldn't do it. But car components have tollerence allotments so it can take it. Just use common sense

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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.

share|improve this answer This will happen when you're driveing diffrant size of tire on an awd. And 4wd cars that have 4high and 4low locking diffranciAls

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Had the same noise and problemen on 4h and 4low with my opel campo 3.1l 4wd because the gear ratio on the front diff wasn't maching up with the rear diff when driving with differant tire size . When i change all 4 tires with the same size everything just works fine. b

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