Need a tire? It may be best to buy two

Dear Car Talk: I have a 2015 BMW X3, which is equipped with run-flat tires. My tire tread was measured, and I was told I have “7s” on the front and “9s” on the back. However, I recently got a flat tire on one of my rear 9s. So I bought a brandnew tire and put it on the front, along with the other 9. 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 intentionally deflated the new tire a little bit (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 Answer: 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 of diameter control, anyway.

Let’s start with the basics, Chen. When a mechanic measures your tire and says it’s a 9, he’s not talking about how sexy it is. 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 before they get that worn out.

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-wheeldrive 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.

Add to that the fact that you are endangering your all-wheel-drive system by using tires of different sizes. Your X3, like most all-wheel-drive vehicles, has a center differential. That allows all four wheels to turn at different speeds (which they must do) when the car is turning.

But if you have different size tires on the car, the wheels will always be turning at different speeds, adding lots of wear and tear to the differential. And center differentials are expensive, so you don’t want to risk yours unnecessarily.

So what do you do now? Well, manufacturers have different recommendations about how similar tires should be to one another (check your own owner’s manual). But most suggest a tread difference of no more than 2/32 or 3/32 inch. So if you’ve got two 7s and a 12, you’ve got a problem, Chen.

One solution is to simply keep the new tire you bought, and buy three more. That’s expensive, because you still 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 to most manufacturers.

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!