Just found a big screw in one of my front tyres. Looking at it it might just be on the edge of the repairable section. If it can't be repaired can i get away with only changing the 2 fronts or do i have to change all 4. Tread on all 4 is down to 3.5mm. Also if i use the space saver is it better to put it on the rear axle? Car is an S3.