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Do I have to replace all 4 tires at once?

Should you replace all 4 tires ?

Is it ever ok to replace one tire?

Not usually because your tires should match as much as possible, for your vehicle to perform at its best and keep you safe on the road.

But there is one exception - when all of your tires are new and only one is irreparably damaged, then replacing that tire is absolutely fine. But try to replace like for like - the same tire (brand and model) and of course, the same size. 

However, most of the time, replacing just one tire is a bad idea, including in these situations:
  • If you drive a car with all-wheel drive (AWD)
  • If the tread on your other tires is below the legal limit (one-sixteenth of an inch)
  • When you can’t find a tire that’s a perfect size and tread match

Can I just replace 2 tires on my vehicle?

There are various issues that arise if you are thinking of purchasing only two brand new tires, especially if you own a four-wheel drive car/AWD. No matter where you decide to place the new tires (i.e. front or back), there is evidentially a substantial amount of risk involved. If you decide to replace the rear tires, you will lose traction when cornering and turning in the back end of your vehicle, which causes a risk of sliding off the road due to oversteering. This is clearly a highly dangerous manoeuvre and if done at high speeds could cause significant damage to the vehicle and harm to those in it and around them. On the other hand, if you decide to replace your front two tires, you will become subject to understeer and may find it harder to turn around sharper bends or corners. Either of these scenarios are far from ideal for any road user so justify the need to buy all four tires at a time.

Uneven wearing of tire treads 

Evidently, if only replacing one or two tires at a time, your current tires along with the new tires will now have mismatching tire treads. This can be problematic for many reasons.

Firstly, if one of your tires has worn out significantly more, this will force the vehicle to try and shift the torque of the car to the tire with the most traction, which will also lead to said tire becoming worn far quicker than it usually would, therefore overall increasing your costs as this will become an ongoing process until you decide to buy four new tires at once so the tire tread in each is balanced as it should be.

This also highlights another key point that in the long term, buying four tires at once is more cost efficient because not only will you step out of the trap detailed above, but avoid any cost of damage done to the car due to oversteering or understeering due to uneven tread depths.

What about four-wheel drive cars/AWDs ?

The idea of not buying four new tires for a four-wheel drive car is fundamentally incorrect. The automatic system that exists under an AWD car requires the technology to be able to use whichever tire has the most grip and traction with the road at any given moment. Therefore, if there are inconsistencies with the tread depths, this can cause the system to not act as it should, as It may not be able to provide as much torque as it should be able to. This could lead to far greater problems in the long run for your vehicle, since the car will be trying to produce more torque than it can use, which will not only damage your four-wheel drive system, but also damage your differential in the long run. Now this can lead to further cost increases because you decided not to buy a set of four new tires in the first place.

Does it depend on the type of vehicle you drive?

Because of the various disadvantages already outlined in buying just 2 tires instead of a full set of 4, these will apply to every road vehicle (SUV, truck, family car, smaller cars etc.) In particular, the costs and potential hazards of having uneven tire treads is not worth the money to save, and in the long run may even cost you more financially. It is widely commented online that buying a full set of tires at a time is much preferable, largely because it is not only cost efficient, and safer than buying just two, but also easier for the consumer in terms of convenience and time or effort required. 

So in summary, most of the time it's usually best to replace the full set of tires.
If you’re concerned about the cost of replacing all four tires at once, then maybe look at an extended auto warranty with wear-and-tear coverage. These warranties often cover the cost of tire replacement.