New tires

I recently took my 2003 MDX to a tire store where I was informed I could not purchase new tires 2 at a time because it is an all wheel drive vehicle. Two of the tire are still good but two need to be replaced. I was told I’d do some sort of damage to the car transfer something or other if I had new tires mixed with older tires. This is my first SUV and the tire prices are a bit more than I anticipated. I’d like to buy two tires now and two more in a few months. Am I risking tearing up some crucial element of my car?

Yes. You need 4 matched tires. If this is too expensive, you could get a fwd Pilot, say.

Agree with @texases and the tire store. An AWD is very expensive to repair and you will need repairs if all 4 tires don’t match. You are very fortunate that they did not sell you 2 new tires and send you on your way…very fortunate.

“Two of the tire are still good but two need to be replaced.”

You need to rotate your tires more frequently then you do so they wear at the same rate. Now you know the added expense when you don’t. Most tire places offer free rotation with the road hazard warranty. They will then wear more evenly. Not only is not rotating hard on the tires, it’s hard on the drive train of an AWD car. The benefit is that awd has the potential to make tires last longer then fwd cars. Tires need to match diameter, tread depth and design. But, in reality ALL cars should be treated the same for safety.
You are risking additional wear already on your drive train by having two tires that need replacement and two that don’t. It is safer to buy four new tires now and delaying or only getting two will be harder on your drive train.

In addition to the valid comments made by the other forum members, I want to add that there is one additional option, namely to buy two new tires and have them “shaved” so that they match the tread depth/circumference of the older tires. This means removing many thousands of miles of rubber, but it is worth it in order to avoid the inevitably huge repair costs that will result from using tires that are not “matched”.

Many–or more likely, most–tire stores do not have the equipment necessary to “shave” the tires, so this does drastically reduce your choice of tire stores.

However, if I was in your situation, I would do the following:

Buy 4 new tires of a high-quality brand (I prefer Michelin, others have their own favorites).

Resolve that you will faithfully rotate your new tires according to the guidelines in your Acura’s maintenance schedule (This could be every 5k miles, or every 7.5k miles, or perhaps every 10k miles). Clearly, you have not been doing this, and as a result, you may already have done some damage to the center differential, but at least you can prevent further/future damage if you begin to maintain the vehicle as it is supposed to be, and on an AWD vehicle that includes tire rotation on a consistent basis, at the same mileage interval each time.

Is the MDX an all time AWD vehicle? or Is it like the RAV4, where it only kicks in when it detects slippage. If it is the latter, you can get away with just replacing tires in twos.

The RAV uses a full time torque management center differential. It is always engaged. In early Honda CRV which it needs to be compared to, there is slippage before it engages. Regardless, Once engaged, the tires need to be same diameter. The MDX is more advanced then the CRV, which means it’s engaged all of the time too. It has been discribed as a variable torque management system too…which means, it’s always engaged. Some have the wrong perception of most Awd units. They may be fwd bias, but the vast majority, other then early crvs, are ALWAYS ENGAGED.
So, when torque, no matter how little and as low as 10% is going to the rear wheels, there is more stress if the diameter if the tires differs. This mistaken idea is from observing that the front wheels on older cars do slip first, but only because there is more torque going there, and not none.

Let me add one thing. These systems are pretty rugged and it isn’t like it will self destruct when you drive out of the shop with only two new tires. But, everything you do, whether loading it to improper or different tire inflation pressures to different tread design to different diameter increases the wear. So, if it’s new and you trade every two years, go for it. If it’s a 2003 with a lot of miles, I would be more, not less careful. My experience with others and my own awd, 4wd vehicles, is that the first 100K are pretty uneventful regardless of what you do. Then to the next 100k and beyond, it’s more critical what you did the first 100k and later. Like eating junk food and smoking and getting away with it when your young then get heart decease or diabetes or lung cancer when you are older, even if you have a lifestyle change at 50 !

This is also a good warning about your current tires. Since the front tires wore down far faster than the rear tires, you have a size mismatch now. Rotating yor next set of tires at least every 10,000 miles will prevent this size mismatch and put less stress on your AWD system. If you plan to keep yor MDX over 100,000 miles, this will help keep our transmission in top shape as long as you own the MDX.

So, I read these remarks and everyone pretty much agrees that the differential and transfer machinery will wear quickly if the tire diameters are different (which causes the wheels to rotate at different rates). What about those of us that live where the roads are winding? The four wheels are very rarely all rotating at the same rate, because I’m always going around a curve. Does that mean that the 4-wheel drive system is not up to the challenge of living in New England or any mountainous region? That doesn’t make sense. That’s where four wheel drive is the most needed. So why is it that these vehicles don’t just grenade after a couple of years? Vermont has winding dirt roads everywhere, and there’s no way the four wheels all turn at the same rate very often, but Subarus are everywhere there and have a loyal following.

I don’t understand this phobia about tire variations.

On average they travel the same distance, unless you drive in a small circle constantly…

“This phobia” results from numerous folks over the years complaining that their expensive awd components have failed, only apparent cause: different tires.

—wentwest : I don’t understand this phobia about tire variations.—

It’s always seemed kinda goofy to me too, since even with brand new perfectly matched tires at identical inflation, you can easily see the front ones more flattened out than the rear, due to engine weight. Still, here is one manufacturer’s recommendation I found on the Web:
Subaru — Within 1/4-inch of tire circumference or about 2/32-inch of each other in remaining tread depth.

You are right. Just about everything stresses or works a differential whether it be a center or rear differential on a rear drive truck. Think of it this way. It’s the secondary gears or differential gears that have to turn in an axle differential to make a corner. If the tires are unequal they will be turn all of the time in addition to the corners and faster then they should on some turns. It’s the additional wear that unequal diameters create all of the time and not just when taking a corner. What ever system is used to accomplish the same thing on an open center differential between the front and rear will also have to work harder if the tires aren’t the same diameter.

And yes, some terrain does wear out differentials faster. And, it isn’t just a phobia. It actually happens. It happens in ordinary cars when tires of different diameters are put on the same axle. They wear faster as well. It isn’t just an Awd phenom. It happens with all cars whose unequal tire diameters share the same differential.

– Auto-owner: It’s always seemed kinda goofy to me too, since even with brand new perfectly matched tires at identical inflation, you can easily see the front ones more flattened out than the rear, due to engine weight.

Sort of.

Rolling diameter is not something you can see. The difference in deflection as you pointed out does cause a difference in rolling diameter, but it is not as much as you might think - and certainly not as much as it looks.

The problem is that different tires will have different rolling diameters even though the size is the same. Sometimes the difference is much, much larger then the difference between individual tires of the same make and model. Since a tire shop can not be sure what the differences are going to be, they have to default to a “safe” position - which is replacing all 4 tires.

CapriRacer, I agree with you completely except for using the term “rolling diameter”. I would have used the term rolling circumference. I know that is nit picky but to me it is important, but then I don’t work in the tire manufacturing industry so that may be a common term there.

keith, rolling diameter and rolling circumference are both different ways of saying the same thing - that pesky pi thing.

And it doesn’t matter. You can’t measure either of those directly. You can measure the rolling circumference by measuring the length of road surface traveled in one revolution - or by counting the number of revolutions per mile - but you can’t just measure anything directly that gets the value. For example, if you measure a freestanding tire’s circumference, you will be about 3% too large.

The circumference of a tire remains constant regardless of deflection due to weight. The diameter may change but not the circumference.

If the diameter of a circle changes, the circumference will change. C= ( pi) D . But the tire is not a true circle when it is mounted under load on a car. It’s like measuring the capacity of a metal can and then distorting it by denting it. You change the volume of the can. So, though we talk about the circumference of a tire and it’s diameter, when deflected, continuing to use that reference to a true circle is incorrect. Read Tire Rack on the subject. The rolling circumference does change a little when the tire is under inflated.

The point is, according to them, the differential gears responsible for this difference must now work over time and continuously regardless of how slight it is, instead of momentarily as they would in a corner. This generates more heat and wear. Obviously, the greater the speed, the greater the wear. Uneven tire wear just exacerbates the problem with the differential gears causing them to work even harder. The idea, is to minimize the number of factors that induce additional wear.

Difference in tire wear is a factor that can add wear over the life of a car for thousands of miles while differnence in tire pressure may be a temporary factor. But, make no mistake. If you put the two together, along with a heavy uneven loads and higher speeds, wear overall is greatly increased.

Or find 2 reasonable priced used tires on CL. Pretty easy. I just got 4 winter snow tires with 11/32 tread for $50