I only need 2 new tires. Should I buy all 4?

Q. I just replaced the front tires on my 2006 BMW 330xi. The tire shop said that since the rear tires are 5/32, there is too much difference in tread depth from the new front tires, and that running the car with two new front tires and half worn rear tires would damage the transfer case. Do I need to replace the rear tires as well? I looked in the owners manual as well as my BMW repair guide and found no reference to replacing all four tires at the same time. What happens if I get a flat on one tire, do I need to replace all four?

A. Looking at other makes and models of AWD vehicles, most indicate that if the difference between the tires is greater than 4/32, it is possible that over time the drivetrain will be damaged. If this were my car and I planned on keeping it for a long time, I would replace the other tires and sell the used tires. If you damage one tire and the other three are in good shape, you can buy a new tire and have it shaved down to the tread of the other three. Also, as a side note, when replacing only two tires, the new tires should be placed on the rear of the vehicle, not the front.

Q. Like most people, I’m often online and see people doing product reviews, especially on TikTok. When you, the Car Doctor, mention a product, are you paid by the company?

A. No, I have never been paid to review a product, even if I have been offered to do so. I do receive products from time to time to review. I will mention products that work as advertised. In a previous column I mentioned the Garmin dash cam. Over the years I have purchased Garmin GPS units, I like their quality and customer service, and based on that I decided to use their dash cam.

Q. I had a nail in one of my tires. The local tire store (big chain) pulled it out and put a plug in. Do they also put a patch inside the tire?

A. I assume that since the store is a chain, they follow professional recommendations for tire repair. The recommended repair when the tire is still usable is a plug/patch combination. A plug without a patch is considered a temporary repair.

Q. I am considering selling our 11 year old Nissan with 70,000 miles on it. I have heard of buyers coming back weeks or even months later demanding that the sellers pay for repairs. Am I protected when selling a used car?

A. Selling used cars privately is a very cautious business. There are no implied warranties when you sell a car privately. In states that require inspections, there are specific rules. In Massachusetts (where I live), you can void a car sale if:

• It does not pass inspection within 7 days of purchase date (not registration date).
• Estimated costs of defects or repairs exceed 10 percent of the purchase price.
• You complete the appropriate steps detailed in the “How To” section in the event of a failed inspection within 14 days of the sale date.

This law applies to sales of cars and motorcycles purchased for personal or family use, regardless of mileage or age, by both dealers and individuals. To protect yourself, for example, if you know the car needs tires, brakes, or has a check engine light on, I would note it on the bill of sale. Since these items would prevent the vehicle from passing state inspection, you and the potential buyer would be on the same page.

Q. I’ve seen a lot of people criticize Harbor Freight tools, especially the cheaper Pittsburg brand. I tinker with my family’s cars all the time and my tool budget is somewhat limited. Are these tools worth it?

A. Harbor Freight hand tools can definitely do the job, especially for a DIYer. Even when I was working as a professional mechanic, there were certain tools like flare nut wrenches that I would only buy from Snap-On Tools because they would never round a brake line. Then there were other cheaper brands that worked just fine. Recently, I needed a 5-millimeter Allen wrench. I bought a $15 socket set from Harbor Freight. It did the job perfectly and has a lifetime warranty. Are these tools for professional technicians? It really depends on how often the tool is used and what it is intended for.

Q. I am the original owner of a 2017 Mazda CX with approximately 80,000 miles that has been dealer serviced. Everything I can, I do myself, both to save money and for the personal satisfaction of doing so. Recently, I have been hearing a rotating whirring noise when driving on the highway. I have changed the brakes, front rotors, front wheel bearings, and recently changed the differential fluid in the front and rear. Unfortunately, this noise is still present when I drive the car. I feel a small vibration in the steering wheel when I stop, obviously after applying the brakes. My question is, what am I doing wrong? Could a tire be bad? Or something out of balance? I am at the end of my rope with this situation. This vehicle has served me well for the past seven years and has never given me a single problem.

A. Vibrations when stopping could be caused by a warped brake rotor or even a warped front hub. If it is a hub, it could also be a noise. Could it be coming from the tires? Yes, at this point the easiest test would be to spin the tires. If the noise travels, you know it is tire related. Even a moderate amount of tire scalloping can cause road noise at high speeds.

John Paul is AAA Northeast’s Car Doctor. He has over 40 years of experience in the automotive industry and is an ASE Certified Master Technician. Email your Car Doctor question to [email protected]Listen to the Car Doctor podcast on johnfpaul.podbean.com.