their sizes -- one being used of course the other new -- violate the Porsche tire guidelines for mixing new and used tires on the same axle.
Here's some cut/paste from my digital factory manual:
- 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 %!
(As an aside, when the right front tire of one of my cars had to be replaced due to some impact damage (deer) the insurance company paid to replace both front tires even though the wear was about 15%.)
Here's more:
- Repairs on ZR tyres are impermissible, as is the use of inner tubes in tubeless tyres!
Many (including me...) have elected to violate the above guideline. I rely upon the tire maker for what it considers ok to repair and many say if the tire is in otherwise good shape, if the hole is small (quarter inch or less in diameter) and in the hole is in the tread the tire is repairable with no compromise in the tire's speed rating. Now finding an indy tire shop that will repair an otherwise good tire on one of these cars is hard enough, but finding a Porsche dealer that will fix a tire... Forget it. (However, I had one dealer help me by having another (non-Porsche) dealer under the same corporate owner fix a tire once in a pinch when I was on the road. No names. Oh, and didn't even charge me for the repair even though I offered to pay.)
- 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.
Many elect to 'violate' the last guideline. I have to point out that different tire brands even though the sizes are the 'same' can have different effective rolling diameters and on some models (depending too upon the tires involved) not following the last guideline can cause problems with the ABS or PSM. In AWD cars mixing different tire brands front and rear with different effective rolling diameters which can cause grief to the AWD system.
So, it looks like you're facing replacing both rear tires, at least.
Sincerely,
MarcW.