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Our customers have told us repeatedly of automotive professionals insisting that since their vehicle was all wheel drive or four wheel drive that they had to replace all four tires at the same time. Though this is true on some vehicles, it is by no means universal. The short answer is to check your owner's manual, but in this article we'll give you some more guidelines and share what we know about the subject:
It should go without saying, but all wheel drive and four wheel drives vehicles should have the same tire size on all four wheels. Having different sizes on some four wheel and all wheel drive vehicles can actually do damage to the vehicle. Since tires vary from brand to brand, and even from model to model, damage can even occur with tires of the same size if they have a large enough difference in circumference. However, since this second issue is one of the main focal points of this article, we will be explaining that in more detail under the four wheel drive and all wheel drive headings. There is only one exception we've ever seen to this rule. The BMW X5 actually comes with option packages featuring different sizes front and rear. Even then, the two tire sizes are nearly identical in diameter.
If, for whatever reason, you have two different tread designs on you AWD or 4WD, the tread should at least be similar in type. All season with all season, all terrain and all terrain, mud with mud. We haven't really seen issues with mismatching tread designs, so again the real issue will be with the circumference of the tires, which will be explained in more detail in the all wheel drive section.
Four Wheel Drive Vehicles
Four wheel drive vehicles can be roughly divided into two different types; those with manually locking hubs, and those with automatic hubs. With manually locking hubs, (the type where you have to go to each front wheel and engage the four wheel drive at the axle), you can get away with more. The reason for this is that the front wheels are not engaged at all until you turn the hubs to the "on" or "4WD" position. You can even use different sizes, although you really wouldn't want to use the four wheel drive system while you had vehicle in that configuration. With automatically locking hubs, (ie, the type engage from a switch inside the vehicle) you need to be more careful about the tire matching. That is because with these systems, the front axles are always turning and they disengage at the transfer case, (or 4WD transmission). With any of these systems, how tight their tolerance is, is on a case by case basis. Some are touchier than others, but most of them being fairly forgiving. Two things are definite though: you should always use the same size tires on all four (excepting the aforementioned BMWs) and; you should check your owner's manual to be safe. Usually, if the vehicle requires close matching of the tires, the owner's manual will be very clear, and probably very loud about it with warnings. As an example, we had a Jeep Cherokee in the other day, and there were "warning!"s everywhere about all sorts of things, but nothing about having to replace all four tires at the same time, or even about tire matching.
All Wheel Drive Vehicles
All wheel drive vehicles are different from four wheel drives in that all four tires are engaged at all times. These range from the extremely tightly toleranced to the more forgiving. Again, the owner's manual is always a good place to start. Some good examples of these are as follows:
Subarus: Subarus have, to our knowledge, the tightest tolerance of any AWD system. Since we see so many, we can tell you what the tolerance is. It can be stated in two ways: The first is 2/32nds tread depth across all four tires. The problem with this is that actual tire dimensions can vary from brand to brand and even from model to model. Therefore the 2/32nds rule is only good if you have the exact same size, brand, and model tire. The other way to check is to measure the tire around the circumference and then the tolerance is 1/4". That was circumference, not diameter, so you have to have a flexible tape ruler and measure around the tread of the tire. Also, having air in the tire affects the circumference. Though the change due to having air in it is only about 1/8", it's pretty critical since the tolerance is only 1/4". Therefore, to get an accurate measurement, it's necessary to let the air out before you measure (since the prospective replacement probably won't be aired up). This second method allows you to check compatibility with any make or model of tire. When we were almost finished with this article, a Subaru came in that needed only one tire, so the picture shows us measuring the prospective replacement tire with our "Subaru Tool".
You may ask what the big deal is? Well, on the Subarus, if all four tires aren't matched within 1/4" around the circumference, you will break the transmission, not maybe or sometimes - it definitely happens. Granted it won't happen overnight, and the AWD on many Subarus can be disabled, but the transmissions can definitely get ruined and then they have to be repaired or replaced. Then the cost will be a lot more than the cost of tires!
Audi Quattros: Audis are another common AWD that we have the information on. Their tolerance is 4/32nds tread depth. Again, unless you have the exact same brand, model and size of tire, this isn't useful. So measured around the circumference, this translates to about 1/2". Again, this has to be measured with the air out, since with air in the tire, there will be around 1/8" difference. We don't see quite as many Audis as we do Subarus so we're not sure how severe the damage to the vehicle would be. However, Audi definitely makes a big deal about this, so it's better not to risk it.
Other AWDs: As stated at the beginning of this article, there is no pat answer for all cars, so the best thing to do would be to check the owner's manual. As an example, we had a Chrysler Pacifica AWD in and we checked it's owner's manual, it simply stated that all four tires had to be the same size. That is definitely more forgiving than Subaru or Audi, but again, different vehicles will be different. We've seen other vehicles whose owner's manual did say that all four tires had to replaced at the same time, so it's best to do your homework and avoid costly repairs.
Automotive Professionals: Another way to find out the tolerance for your specific vehicle would be to call your dealer or mechanic. It is very possible that a mechanic who specializes in a specific vehicle would know the answer. However, the problem here is that people (especially males!) will often give you an answer without actually knowing, so caution is advised.
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