Had a flat tire over the weekend and due to where the cut occurred, they couldn't patch it. The cut wasn't on the sidewall, but right on the edge of the treads (on the shoulder?). Instead of replacing the tire, I opted to put a plug in the damaged tire (it's holding air) and put the spare in service since it's full size, and use the damaged tire as a spare. My question is what is the threshold on when you should replace all four tires due to tread variation? The spare has never been used so it's basically new, and the other tires have 49K miles on them. A new tire has tread depth of 11/32. I just went in for an oil change and the tires were measured at 8/32, so there are 3/32 difference between the spare and the other three tires. I spoke to the service department at my dealership and they told me that I shouldn't be concerned as the variation isn't that great. Do any of you have any insight? I'd hate to have to buy four new tires since they all have quite a bit of life left, but I also don't want to damage the driveline. Thoughts?