Why change all four tires at the same time with AWD?

Question

My current car (AWD Subaru) has new front tires and used rear tires. Why is this bad and what should be done about it? What if I do nothing?

Answer 1

Wow, nobody got this quite right.

The issue has to do with your AWD system and Subaru are more strict than most in their requirements. All four tires need to be not only the same size, but the same brand and model the total difference in wear between them should not be greater than 20%, or about 2/32" tread depth.

Why? Because the minor differences in diameter between mismatched tires can cause the fluid that lubricates the viscous coupling in the transfer case to overheat, causing very expensive damage. I have witnessed this myself on several occaisions.

What happens if you do nothing? You may end up paying for a new transfer case.

Tires are cheaper.

Here is what is says on Subaru's web site...

"For safe vehicle operation, SUBARU recommends replacing all four tires at the same time.
WARNING: All four tires must be the same in terms of manufacturer, brand (tread pattern), construction, degree of wear, speed symbol, load index and size. Mixing tires of different types, sizes or degrees of wear can result in damage to the vehicle's power train. Use of different types or sizes of tires can also dangerously reduce controllability and braking performance and can lead to an accident."

They also recommend that if you have to use the doughnut spare that you deactivate the AWD system since the spare is smaller than the other tires.

ASE Certified Automotive Service Manager working in the tire industry

Answer 2

the priority with AWD autos, as others have spoke of, is that the motor vehicle is designed to have all wheels rotate an identical quantity on an identical time. If, via positioned on and different circumstances, the diameter of one tire isn't like the others that particular wheel is the two going to be pulling extra, or under the others. this would reason secure practices subject concerns, overall performance subject concerns, or maybe provoke mechanical problems. there is, in all properly designed structures, a undeniable quantity of tolerance equipped in so as that a small difference between wheels can't impression any concern. The query is: how plenty will your motor vehicle tolerate? My suggestion is that, as long as you have under one million/4 inch difference between any wheels, you elect not concern, yet you want to demonstrate screen your positioned on on those generally if the version is larger, and surely, if the version is larger that one million/2 inch evaluate changing each and every of the wheels. however in case you are trying this, keep in mind which you do not ought to throw out the tires you replace. shop them in storage until your new tires positioned on down an identical quantity then initiate putting them into your rotations so as which you ultimately use them up.

Answer 3

New fronts and used tires in the back are not a problem if the road is dry, this is why racers use slicks or shave their tires. The problem is if its wet or slushy and you want to stop or steer. The newer tires will have more traction and stay planted while the back will want to come around. A while back there was a question from a guy who found himself going backwards on the highway in the rain scraping against the guardrail with his late model BMW, all the stability controls will not overcome the laws of physics, put the better tires on the back ALWAYS.

Answer 4

AWD vehicles have a clutch in the transfer case. if the rear tires are a different size than the front tires, it makes the clutch slip all the time. Changing all 4 tires gives you the same circumference on each tire and it is easier on the driveline of the car.

Answer 5

Depending on your car and the stability controls that it has having old rear tires can cause your car to oversteer (rear end swings out) badly. If you are in snow or ice the rear of the car will want to come around constantly.

If this is a brand new car it probably has all kinds of stability controls to prevent this. However, under the right (or wrong) conditions even the best driver aids won't prevent a spin under extremely hard braking, cornering or acceleration.

Answer 6

Your tyres should all have the same circumference. This way, when each wheel has turned one full revolution, each will have travelled the same distance. If the tyres are different, the smaller wheels will have to turn faster to keep up with the others. The centre differential can make up for this difference in speed (its job is to do so when cornering), but if it happens all the time (and not just on corners) it will wear out the centre diff very quickly.

Also, since you have power to all wheels, it can cause some very strange handling. Always change all 4 tyres at the same time - and if they had been rotated when they were supposed to, they would have all worn out at the same time.

Good luck!

Answer 7

The tires will run at a different speed of rotation if the wear factor on any one of them is too great....and will confuse the ABS as well as cause ger box wear due to spining the spider gears excessively....

Answer 8

There is no harm as long as all your tires have the same numbers on the sidewall. all tires should match as far as size but wear will do no harm. There is no clutch that slips. I don't know where some of this info. comes from. Run your used tires.

Answer 9

Why not - was one on the car less time than the others ??