All-wheel-drive vehicles need all four at once — the system reads any tread-depth difference between axles as wheel slip and puts strain on the transfer case over time. A front-wheel or rear-wheel-drive car doesn't have that problem and can usually go in pairs, matched by axle, instead of replacing all four.
Why does an AWD system care about tread depth?
An AWD or 4WD system constantly compares how fast each wheel is turning. A new tire has a slightly larger rolling diameter than a worn one, so pairing one new tire with three worn ones makes the system think a wheel is slipping — because, mechanically, it is.
Over miles, that mismatch loads the transfer case and differential in ways they weren't built for. It's a slow failure, not an immediate one, which is exactly why it gets ignored until the repair bill is much bigger than four tires would have been.
What if the car is front-wheel or rear-wheel drive?
A two-wheel-drive car doesn't have that cross-axle computer watching for mismatch, so replacing in pairs — matched to each other on the same axle — is standard practice, not a compromise.
The usual guidance, regardless of which axle is driven, is to put the newer pair on the rear. It keeps the back end more composed in wet conditions than putting fresh tread up front and worn tread out back.
Can I pair tires from two different brands?
The requirement is a matched tread pattern and diameter across the axle, not a matched brand name. A shop can usually find a comparable tire from a different maker if the original brand isn't in stock — the pairing has to behave the same, not be labeled the same.
Is it illegal to run mismatched tires?
No general law bans it outright, but on an AWD vehicle it can trip the stability or traction-control system and, over time, damage the drivetrain — which tends to cost more than the tires would have.
Which two tires go on first if I can only buy two?
The rear axle, by standard guidance — new tires in the back keep the car more stable in wet or slick conditions than new tires up front.
Does rotating tires let me avoid buying four at once?
Rotation slows uneven wear but doesn't reverse it. If three tires are already significantly more worn than a fourth, rotation won't close that gap.