Why it’s best to get two tires replaced when you get a flat

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 that I have "7s" on the front and "9s" on the back. However, I recently got a flat tire on one of my rear 9 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 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? Is it recommended? – Chen

RAY: 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 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-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.

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!

Car’s intermittent dying is probably due to bad fuel pump

Dear Car Talk:

We have a 2009 Honda Odyssey LX. At 90,000 miles, we changed the timing belt and water pump before a cross-country trip. Mistake. On the interstate, driving on the way to the Black Hills, the car lost power for a few seconds, half recovered and then died. It was as if we had run out of gas, but there were two to four gallons left in the tank. We rolled to a stop, turned it off, turned it back on and drove 10 miles to the next gas station, where we filled it up, and it was fine. A day later, the same thing happened near Little Big Horn. Three months later, same thing in Idaho. And then two more times since then over the past year. It's always when it has less than a quarter of a tank of gas in it, and it's always when traveling at or near highway speed. And it always restarts right away. Two Honda dealers couldn't find anything wrong. One dealer suggested that we drain and drop the gas tank, but said that could be a wild goose chase, so we haven't done it. What should we do? – John

RAY: You probably should order a new fuel pump, John. This sounds like a classic case of a fuel pump going bad.

When the fuel pump is weak, it’s most likely to misbehave when it’s hot and been running for a long time, and when the demand for fuel is greatest. When is that? When you’re on a long highway trip, climbing a mountain, and it’s midsummer.

Have your shop put a pressure tester on your fuel pump. I’m pretty sure it’ll be below spec, and a new fuel pump will be the answer.

And, by the way, changing the timing belt and water pump before that big trip was not a mistake at all. In fact, if you hadn’t changed the timing belt, you’d probably be writing us from Little Big Horn to ask how much you should pay for a new engine.

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