A few days after we bought our 2012 BMW Z4 for testing, we noticed—thanks to the car's tire-pressure monitoring system—that the left rear tire was losing air. The tire would start the day at its recommended 44 psi, but by the next morning it was down to about 34 psi. Somehow, 10 pounds of air a day was going missing for no obvious reason.

We took the tire/wheel combination off and inspected it, but our technicians didn't see any damage. Further, they couldn't find any obvious leaks in the tire, from the valve stem, or the bead where the tire and wheel come together. The conclusion: it was likely a slow leak in the tire overlooked during inspection. So we decided to purchase a new replacement 17-inch low-profile Bridgestone Potenza run-flat tire, which set us back $216.

But the new tire also started losing 10 pounds a day. Now stymied, David Van Cedarfield, our most senior shop technician, submerged the tire and wheel, and he caught a trace of tiny bubbles rising through the water. (See the leak demonstrated in the video below.)

Incredibly, the bubbles were coming from the center of the alloy wheel itself, escaping by a pinhole about midway in the wheel. So all along it had been the wheel, and not the tire, that was to blame.

The local BMW dealer replaced the wheel at no charge. Perhaps if we brought the car back for service, the dealer would had identified the leaky wheel as the culprit and saved us the cost of buying an additional tire, but that's fine since we're probably going to wear through a set of tires as we test the car. (Oh, baby...)

Our mechanics have seen all kinds of tire and wheel damage, but never a leaky wheel on a brand-new car with only 2,000 miles on the odometer. We are assuming that our leaky wheel had a casting flaw. Meanwhile, we've seen online discussions about repairing such leaks by applying a sealer inside the wheel. But we don't think leak repair attempts are such a great idea. If the wheel is leaking air, it could be a sign of a serious flaw that no one should be driving on.

Our advice, as always, is to check your tire pressure routinely, and if your car has a tire-pressure monitoring system (like ours), don't ignore the warning—get the tire checked out.

See our tire-pressure gauge buying advice and Ratings.