mixing tires

 

Never mix tire types or sizes. You can create a dangerous instability in the handling.

 

The authority on tires for these cars -- Porsche -- says to not mix tires even they same brand, do not mix axle to axle and certainly do not mix tires on the same axle.

(Heck even if the two tires on the same axle are the exact same tire if the tread depth difference between the new tire and the "old" one on the other side is 30% or more the "old" tire should be replaced.)

Here's some cut/paste stuff from the 996 Turbo factory manual regarding tires but the same "rules" apply to other models as well:

If new tyres are to be mounted or the tyres of one axle are to be replaced, tyres of the same make and with the same specification code must always be used on each of the two axles. If tyres are replaced on one axle only, the different tread depth than on the other axle can cause a noticeable change in the familiar handling. This is especially the case if new tyres are mounted on the rear axle. This effect decreases with increasing tyre mileage.

Danger of injury and damage to property as a result of modified handling behavior if tires are used which do not correspond to Porsche−specific requirements!

When replacing a tyre on an axle, make sure that the tread depth of the new tyre does not differ from that of the other tyre by more than 30 %!

Tyres should not be older than 6 years! Not even the spare wheel!

When replacing summer tires, pay close attention to the tire specification number! The specification characters N1, N2, N3 or N0 distinguish the summer tires specially authorised by Porsche from other models of the same tire type and size!

 

I wouldn't mix them on the same axle per say, however, I'm running Potenza RE050A on the front and Potenza S-04 Pole Position on the rear. I opted for the latter given the edges are rounder and given my low ride height they now don't rub. No issues otherwise.