the MAIN circumstance where diameter is critical is with AWD or 4WD systems, AWD in particular. Those you MUST rotate and replace 4 at a time or your coupler (whatever kind it is) will go crazy. Audis are the MOST sensitive, since their front to back coupling is purely mechanical.

others, which might use a fluid coupling, still have big issues with front to rear circumference variances.

in front or rear wheel drive, it doesnt make much difference unless you are a paricularly anal engineer. main thing is to assure left/right matches, particularly when there is a differential involved. granted, cars with traction control and electronic stability control are electronically monitoring wheel motion around 2000 times per second and CAN get confused. older cars are pretty forgiving.

rich in NC