Walk into a chain tire shop with one bald tire and they'll quote you four. They're not always wrong — but they're not always right either. Here is the actual decision tree.
Drivetrain rules everything
All-wheel-drive and four-wheel-drive vehicles transmit power through every wheel. If one wheel's tire is a different overall diameter than the others — even by half an inch — the differential and transfer case are constantly slipping to compensate. Over thousands of miles, that destroys $2,000 worth of drivetrain components.
Most AWD and 4WD manufacturers require all four tires within 2/32 of tread depth of each other. Subaru and several others spec 1/32. Read your owner's manual — it's in the tires section.
Front-wheel-drive and rear-wheel-drive vehicles are far more forgiving. You can replace just the worn axle (front pair or rear pair) without harming anything, as long as both tires on the same axle match.
When a single tire is fine
A single tire is acceptable when all three of these are true:
- Your car is FWD or RWD
- The other three tires are within 2/32 of the new one
- The other three are the same brand and model, or close functional equivalents
This is the case after a road hazard — nail puncture too close to the sidewall to repair, sudden sidewall damage from a curb, etc. A single matched used tire saves you hundreds.
When a pair is right
A pair is right when one axle is worn down past 4/32 but the other axle is still over 6/32. Replace the worn axle, leave the good axle alone — and put the new tires on the rear, regardless of which axle was worn.
That last rule surprises people. New tires go on the rear because the rear tires control yaw stability. A FWD car with worn rears in the rain will fishtail. With worn fronts it will just understeer, which is more recoverable.
When a full set is the right call
- AWD or 4WD with any tire that can't be matched within 2/32
- All four current tires below 4/32
- Mismatched brands and models on the car already (it happens)
- Switching to a different tire model for any reason — winter to all-season, etc.
There is no universal answer, but there is always a right answer for your specific car and your specific tires today. Don't let anyone sell you four when you need one.