Finished my fourth rotate and balance today. The PO of my car had a set of relatively new Kumho Ecsta ASX tires on the Prius and while I noticed that they have "outside" notated on the sidewall, I did not notice that one of my tires was mounted backwards with "outside" facing inwards. I have had the car 20k miles now and haven't had any problems, so my question is should I "just drive it" or take it to the tire shop and have them remount it correctly?
Get it remounted, you paid to have them mounted correctly. It will make a difference in handling wet or dry. Whether you will notice the difference or not is another question , but the tire manufacturer went to the trouble to make the inside and outside (relative to the car) of the tread different for handling. They probably knew what they were doing.
