Must buy 4 replacement tires,not 2, with AWD?
03-07-14 | 12:14 PM
#17
Driver School Candidate
Joined: Mar 2014
Posts: 3
Likes: 0
From: TX
Let me play the devil's advocate:
There are at least three systems that are affected by mis-matched tires. Stability or traction control systems (1) and ABS (2) need closely matched tires sizes to work correctly, as they constantly compare wheel rpm as a datapoint. When the systems intervene, a warning light and/or alarm usually comes on, so you get immediate feedback if you install a tire so badly mismatched that the system is fooled.
The other issue is transfer case or differential action. I have a friend that fried his diff when he "got busy" and left a grossly mis-matched (visually, perhaps more than an inch in height) spare tire on his drive axle, and it took many weeks of abuse before it failed. So it does happen, & I acknowledge that. But let's look at the facts: a well worn tire versus a new tire is going to be about 1/4" shorter. Your tire size will enter in to this, but ballpark, that means the used tire is probably going to rotate somewhere around 6 additional revolutions per mile. At freeway speeds and in a straight line, 8 extra revs per mile means the differential section is going to go through an otherwise un-needed rotation about every 8 seconds. If that is enough to cause the diff or transfer case to blow, simply driving too many miles on a twisty road would also put your diff at risk.
Also keep in mind that most of the voices telling us to replace all 4 tires are from companies in the business of selling tires.
As for the transfer case that was blown immediately after installing a used tire? I'd wager a week's pay that the story is either an urban legend or very skillful fraud on the part of the owner of the car. Or maybe they left the tire dealership and immediately made a couple of passes on the salt flats at Bonneville...
Personally, I wouldn't willingly mix well-worn and new tires on the same axle, but I wouldn't hesitate to run a small mismatch front to rear.
There are at least three systems that are affected by mis-matched tires. Stability or traction control systems (1) and ABS (2) need closely matched tires sizes to work correctly, as they constantly compare wheel rpm as a datapoint. When the systems intervene, a warning light and/or alarm usually comes on, so you get immediate feedback if you install a tire so badly mismatched that the system is fooled.
The other issue is transfer case or differential action. I have a friend that fried his diff when he "got busy" and left a grossly mis-matched (visually, perhaps more than an inch in height) spare tire on his drive axle, and it took many weeks of abuse before it failed. So it does happen, & I acknowledge that. But let's look at the facts: a well worn tire versus a new tire is going to be about 1/4" shorter. Your tire size will enter in to this, but ballpark, that means the used tire is probably going to rotate somewhere around 6 additional revolutions per mile. At freeway speeds and in a straight line, 8 extra revs per mile means the differential section is going to go through an otherwise un-needed rotation about every 8 seconds. If that is enough to cause the diff or transfer case to blow, simply driving too many miles on a twisty road would also put your diff at risk.
Also keep in mind that most of the voices telling us to replace all 4 tires are from companies in the business of selling tires.
As for the transfer case that was blown immediately after installing a used tire? I'd wager a week's pay that the story is either an urban legend or very skillful fraud on the part of the owner of the car. Or maybe they left the tire dealership and immediately made a couple of passes on the salt flats at Bonneville...
Personally, I wouldn't willingly mix well-worn and new tires on the same axle, but I wouldn't hesitate to run a small mismatch front to rear.
10 months ago I bought a 2006 Audi A4 Quattro. It came with 2 different sets with different length tread on the front and the back. I turned off TCS out of personal preference. I am a total noob with cars and had no idea how the different tires could be bad for an AWD car. I got a flat and replaced the rear driver side tire with a brand new full size spare from the trunk and drove it for nearly 2 months.
Today I went to Discount Tire to replace 2 tires and they refused to work on my car unless I had 4 matching brand tires. I ended up going for it because they gave me a huge deal on all 4 tires, but I want to point out that I drove this car like this for over 10 months and 3 different brands lengths for 2 months.
My differential didn't go out, my transmission didn't go out and my transfer case didn't go out. Maybe there is abnormal and significant wear, that could be a possibility, but I would think if this was as serious as they say I should have surely had a problem within 10 months. I also drive very aggressively. I have to assume the risk is much overstated.
03-07-14 | 09:33 PM
#18
Do what you want, its your car... I'm back in the automotive business again and I really pay attention to this sort of thing...
Not too long ago, someone came in with an AWD Ford Explorer with the front driveshaft removed... After asking them why, I found it was when they replaced two tires (same size, different brand) that the vehicle started making crazy noises from the drive line... They didn't realize it could be a tire issue and removed the front drive shaft... After putting a set of matching tires back on and the driveshaft back in, no further issues...
Its obvious it affects vehicles to varying severities, I'm not going to take a chance, that's for sure....
Not too long ago, someone came in with an AWD Ford Explorer with the front driveshaft removed... After asking them why, I found it was when they replaced two tires (same size, different brand) that the vehicle started making crazy noises from the drive line... They didn't realize it could be a tire issue and removed the front drive shaft... After putting a set of matching tires back on and the driveshaft back in, no further issues...
Its obvious it affects vehicles to varying severities, I'm not going to take a chance, that's for sure....
03-16-18 | 09:34 AM
#19
Rookie
Joined: May 2017
Posts: 26
Likes: 4
From: PA
Buying 4 tires for AWD - Learned the Hard way
In 2004, I bought a 2002 Volvo V70 AWD Wagon. While driving from Philly to St. Louis, I blew a big section out of the tire in Indy when a cop without lights ran me off the road. I drove the rest of the trip on the space saver. I only replace one tire. I had catastrophic transmission failure at about 110K miles. I drove about 10K miles with the mismatched tires then replaced all 4 per the manual. This is the recommendation by Subaru, Volvo and others. Since most people dont buy tires from the dealer, there must be a reason. All the transmission places and other people indicate replace all 4. After transmission was replace in Volvo, the car now has close to 375K miles. All four replaced anytime there was a tire issue. While this is anecdotal, it does tend to support the theory. YMMV
03-19-18 | 10:02 AM
#20
Driver School Candidate
Joined: Mar 2014
Posts: 3
Likes: 0
From: TX
Just to throw my hat back in on this one. I drove that 2006 A4 from 81K mi to about 130K. To my knowledge, I didn't have any serious transmission issues. I ended up with massive electrical and engine issues, so I don't drive that car anymore. I ended up getting a Volvo S60. It came with all matching tires, so I'll get them all replaced at once. But I'm still not entirely sold on the reason as to why this happens. I'd like to think that modern differentials and transmissions can compensate for this. It just seems like a real problem to get 1 flat that can't be repaired and have to buy 4 new tires.
05-20-18 | 11:25 AM
#21
Pole Position
Joined: Jul 2012
Posts: 241
Likes: 32
From: LA
For the money involved why take a chance? If you need to replace one tire, measure the tread depth, call Tire Rack, order a matching brand size tire and have it shaved to match your remaining 3. I did this recently on my Audi S-8. Hit a pot hole that caused a sidewall failure. Called Tire Rack, and in this case I was running tires purchased from them 18 months earlier, they had the dealer measure the tread depth and shipped the dealer a new tire shaved to match at no cost to me since the tire was covered under the Tire Rack road hazard warranty.
05-21-18 | 12:53 PM
#22
Driver School Candidate
Joined: Mar 2014
Posts: 3
Likes: 0
From: TX
For the money involved why take a chance? If you need to replace one tire, measure the tread depth, call Tire Rack, order a matching brand size tire and have it shaved to match your remaining 3. I did this recently on my Audi S-8. Hit a pot hole that caused a sidewall failure. Called Tire Rack, and in this case I was running tires purchased from them 18 months earlier, they had the dealer measure the tread depth and shipped the dealer a new tire shaved to match at no cost to me since the tire was covered under the Tire Rack road hazard warranty.
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