Tire Replacement - 30k mile, mechanic says its okay to only replace one?

Question

We have a 2014 Toyota Corolla with a little under 30,000 miles. A few months ago some road debris resulted in a punctured tire (rear passenger side). We had the tire fixed, but found out that they plugged it and didn't actually patch it like we thought. Since it leaks air when it gets really cold, we went to another shop to see if they could replace the plug with a patch, and said we should replace the entire tire.

The mechanic insists that we only need to replace the one tire, saying its only the rear and that the other tire has minimal wear. I know you should normally change tires in 4s or at least in 2s on the same axle. The tires are primacy MXV4, which has a 60k miles warranty.

At 30k miles, I assume that the tires cannot really be like new, even though the car is just for commuting. I am also concerned about how this impacts syncing up with future tires changes for the other tires. I would rather change both and keep the rear driver side as a full spare rather then rely on the donut. I was willing to pay for more tires+service, so I assume if the mechanic is trying to talk me out of it, he has a good reason (why would he not want to make more money?)

Any validity to his point of view? Or should I insist on giving him more money?

Edit: The replacement tire is the same type/size as the stock tires on the other 3.

Answer 1

tl;dr: As this is a front wheel drive car and you're taking about replacing one of the rear tires, it sounds like you'll be fine replacing a single tire.

The key facts of your situation are that:

1. You're driving a front wheel drive car without a differential to worry about in the rear.
2. Your tires are long-lived commute-only.

Given those two points, I'd expect that you're installer is correct that you only need the one tire.

The most important reason is the fact you don't have a differential to worry about in the rear. If your tires have slightly different circumferences due to uneven wear, it might cause a slightly different track down the road but it won't break anything mechanical.

This is in contrast with AWD cars like mine, for example, where all the wheels are connected at some level. On my car, I car hear mechanical complaining if I over inflate the rear tires by a few psi.

If you're worried about the uneven wear, you can easily check it yourself. Take an American penny (or the local equivalent) to the good rear tire and stick the penny in between the tread blocks at the outer edge, the middle and the inner edge of the traction surface. Make a note of what you see: can you see all of Abe's hair? None? Compare each of those measurements with the new tire. Are there blatant differences? Can you see Abe's while head on one tire but his nose is hidden in the other?

If you can't easily see significant wear differences, you are probably fine replacing just the one tire.

All that said, if you were feeling fancy, it isn't the worst idea to replace both of those rear tires and save the worn one as a garage spare for the next time you have one go bad on you. That is, of course, dependent on your garage situation. I currently have four winter tires on wheels in my garage and I've reached the limits of spouse tolerance. Your mileage may vary.