Why you should get two tires replaced when you get a flat

Dear Car Talk: I have a 2015 BMW X3 equipped with run-flat tires. My tire tread was measured, and I was told that I have “7s” on the front and “9s” on the back. However, I recently got a flat tire on one of my rear tires. So I bought a brand-new tire and put it on the front, along with the other 9 that used to be on the back. Then I put the two 7s on the back. Now I’m told that the new tire is bigger than the old 9 tire, and it will screw up my all-wheel-drive system. So I deflated the new tire (29 psi, compared with the recommended 30 psi) and inflated the other 9 (to 32 psi, compared with the recommended 30 psi), and I kept the rear 7s 35 psi, as recommended. The purpose is to make the new tire a little smaller, so that it’s the same diameter as the 9 tire. Is this a good approach? — Chen

No, and no. I don’t recommend toying around with tire inflation, because it can compromise both safety and handling. And it’s not a very effective method, anyway.

Let’s start with the basics, Chen. When a mechanic measures your tire and says it’s a 9, it means that you have 9/32 inch of tread left. Most tires start out with about 12/32. When you get to 2 or 3, you’ll see the tire’s wear bars, which means the tire is legally ready to become a swing. But in reality, most people will want to replace their tires sooner.

Studies show that stopping distances are much longer on wet roads when tread depth gets below about 4/32. And performance on snowy roads degrades below 5/32. And since you bought an all-wheel-drive vehicle, Chen, I’m guessing that weather is an issue where you live. That means those 7s are already getting near the end of their useful lives.

One solution is to simply keep the new tire you bought, and buy three more. That’s expensive, because you have some useful life on the three tires you’d be throwing away. But that’s the best option from a mechanical point of view.

Another option is to have that new tire “shaved” to match its axle-mate. That involves taking a perfectly good, new tire, and paying a tire store $30 to turn it into a tire with 15,000 miles on it. Most people resist that idea because it seems wasteful.

But when you compare that with the cost of three more new tires, shaving or matching the new tire may be the way to go. Then you’d have two 9s up front and two 7s in the back. Not ideal, but acceptable.

Of course, once those 7s wear out, you’ll have this problem all over again. So you may want to look for a tire dealer who gives you a free sandwich with every six tires you buy. Good luck!