Poor quality on those continental tyres

Continental Tyres. I've been using these tyres for about 2 years and they keep disappointing me. Just last week I took my car for a service and VW told me that my tyre had a bubble and replace it with a spare wheel. Okay cool, no problem On Saturday, two of my tyres BURST and when we I look there are bubbles all over.

Right now I put my spare wheel and anything can happen.