Find the code already on your tire's sidewall (or driver-door jamb sticker) and stick to it unless you're deliberately changing fitment. Mismatched sizes affect handling and speedometer accuracy.
The UTQG treadwear number (printed on the sidewall) estimates longevity — higher lasts longer but often trades away outright grip. A commuter car and a mountain-road car want different trade-offs.
Mismatched tread depth or tread pattern across axles changes how the car handles, especially in rain or snow. Two new tires go on the rear axle, not the front.
Look for the 4-digit DOT code (e.g. "2324" = 23rd week of 2024). Rubber ages even unused — avoid tires older than about 4–5 years off the shelf.