We get asked this question a lot. In the old days when cars were(mostly) either FWD or RWD replacing 2 tires at a time was the common practice. The problem is that these days 4WD(four wheel dirve) and AWD(all wheel drive) vehicles are becoming more popular and more predominant. 4WD/AWD vehicles have different procedures when it comes to tire replacement.
What’s the difference?
The difference between a 2WD vehicle and a 4WD/AWD vehicle is sort of technical. On a 2WD vehicle only 2 of the 4 tires actually gets power sent to them while the other 2 just kind of….follow. With a 4WD/AWD vehicle all 4 tires get power sent to them. The computer actually monitors each wheel. It looks to see if the speeds are reading the same so it can look for things like slippage and change the output accordingly.
Why does this matter how many tires I replace?
It all boils down to rolling resistance and wheel diameter. Let’s say you have 2 tires that are pretty worn but the other are still some what ok. 2 things should be evaluated:
- Are you regularly rotating your tires?
- Is there an alignment issue?
Even on a 4WD/AWD vehicle the tires do a different amount of work. Just because you hear “Four Wheel” or “All Wheel” doesn’t mean that tires are going to wear the same. Tire Rotations are especially important on these kind of vehicles because you want to get all 4 tires worn the same way.
Remember when I said “The computer actually monitors each wheel”? That’s where the problem is. There are 2 scenarios that should make it all clear.
Scenario #1: You want to replace 2 extremely worn tires with new ones but leave 2 very worn ones on the vehicle.
The problem here is that the overall diameter would be completely off. New tires usually measure from 10-/32nds – 12/32nds. And say your very worn tires are measuring in at around 5/32nds. Well, that’s half of the new ones which means the new tire will actually roll LESS than the worn tire. Let’s say that maybe the new one rolls 700 times in a mile but the new one rolls 693 times in a mile. It doesn’t seem like a lot but it can completely throw the systems off causing unnecessary wear and strain. When the computer is watching all 4 tires and it sees that 2 of them are rolling at a different rate than the other 2 it thinks there is a problem and attempts to correct it by changing how much torque goes into each tire. Which means that there are 1 or 2 tires that are working harder for no reason. Because of this scenario it is HIGHLY RECOMMENDED that if your car is equipped with a FULL SIZE SPARE TIRE that you incorporate it into the rotation of your tires. That way, if something were to happen and you needed to use your spare, the tire would be worn to match the others.
Scenario #2: You put mismatched tires on your vehicle.
What does mismatched mean? When we say “Mismatched Tires” we mean either 2(or more) brands of tires OR 2(or more) different tread designs. This is where “Rolling Resistance” comes into play. Let’s say that you have 2 A/T(all terrain) tires on your vehicle and you want to put 2 Touring tires on with them. A/T tires have an extremely aggressive/rough tread where as Touring tires have a very smooth/easy riding tread. The new Touring tires will have LESS rolling resistance than the A/T tires causing them to roll at a different speed. Just like it is stated above, this will cause the computer to over compensate because it thinks your tires are slipping.
If you drive with a mix of tires long enough you will eventually burn out your differential/transfer case because of it having to overwork every second you are driving that vehicle. Just remember that if you regularly rotate your tires you wont have to worry about replacing only 2 tires at a time because all 4 of them should be worn evenly.