Originally Posted by avanti
Why not just replace in pairs (front or back)? It is not important that tires on different axles match wear-wise.
I sorta agree with this, but with a bit of caveat. As far as durability against failure, yep, but depending on the wear seen on the older pair, you can get a traction differential that may be hazardous under some conditions.
This was kind of a big deal when front drive cars started to get common here in Minnesota as lots of people were putting snow tires, and/or the best tires on the front so they would have the most traction. Not surprising, there were quite a few front drives doing end swaps on turns when the low traction, low weight, rear let go in the slippery. I think nearly all the tire shops now will only put the best tires on the rear of front drives and rear drives.
The other issue is with AWD/4WD, which needs to have the same rpm going to each end of the vehicle to prevent error codes or part damage from speed difference due to tire diameters. Same as when you put mismatched tires on a limited slip rear or even open rear differential. If the tires a much different, you have the carrier turning on the pinions all the time and can overheat the differential.