I only need 2 new tires. Do I have to buy all 4?

Q. I just replaced the front tires on my 2006 BMW 330xi. The tire shop said because the rears are at 5/32, that is too much difference from the new front tread depth, and running the car with two new front tires and half-used back tires will damage the transfer case. Should I replace the rear tires also? I looked in the owner’s 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 had a blowout on one tire, would I need to replace all four? 

A. Looking at other makes and models of all-wheel-drive vehicles, most state if there is more than 4/32 difference in the tires, it is possible over time to damage the drivetrain. If this were my car, and I planned on keeping it a good 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 one new tire and have it shaved down to the tread of the other three. Also, as a side note, when only replacing two tires, the new tires should go on the rear of the vehicle, not the front. 

Q. Like most people I’m online a lot and see people doing reviews of products, 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 do a review of any product, although it has been offered. I do from time to time receive products to evaluate. The products that work as advertised I will mention. In a past 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 decided to use their dash cam. 

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

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

Q. I am thinking of selling our 11-year-old Nissan with 70,000 miles. I have heard of stories of buyers coming back weeks and even months later demanding that sellers pay for repairs. Am I protected when I sell a used car? 

A. Private party used car sales are truly buyer beware. There is no implied warranty when you sell a car privately. In states that have vehicle inspection there are specific rules. In Massachusetts (where I live) you may cancel the sale of a car if: 

• It fails to pass inspection within 7 days from date of purchase (not the date of registration). 
• The estimated costs of defects or repairs exceed 10 percent of the purchase price. 
• You complete the proper steps detailed in Failed Inspection “How To” within 14 days of date of sale. 

This law applies to both dealer and private party sales of cars and motorcycles purchased for personal or family use regardless of mileage or age. To protect yourself, as an example if you know the car needs tires, brakes, or the check engine light is on, I would note that on the bill of sale. Since these items would keep the vehicle from passing state inspection, both you and the potential buyer would be on the same page. 

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

A. Harbor Freight hand tools can certainly work, especially for a DIY’er. Even when I worked as a professional mechanic there were some tools such as flare nut wrenches that I would only buy from Snap-On tools, because they never rounded off a brake line. Then, there were other cheaper brands that worked just fine. Recently I needed a 5-millimeter allen wrench. I purchased a socket style set for $15 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 gets used and for what purpose. 

Q. I am the original owner of a 2017 Mazda CX with approximately 80,000 miles that has been dealer maintained. Whatever I can, I do on my own repairs, both to save money and for the personal satisfaction of doing it. Recently I have been hearing a rotating whirling sound when I am driving on the highway. I have changed the brakes, the front rotors, the front wheel bearings, and recently I changed the differential fluid in both the front and the rear. Unfortunately, this sound is still present when I drive the car. I do feel a little vibration at the steering wheel when I come to a stop obviously after applying the brakes. My question to you 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 been great to me over the past seven years and has never given me any trouble whatsoever. 

A. The vibration when stopping could be an out-of-round brake rotor or even out-of-round front hub. If it is a hub that could also be a noise. Could it be the tires? Yes, at this point the simplest test would be to rotate the tires, if the noise travels you know it is related to the tires. Even a moderate amount of tire scalloping can cause some road noise at highway speeds. 

John Paul is AAA Northeast’s Car Doctor. He has over 40 years of experience in the automotive business and is an ASE-certified master technician. E-mail your Car Doctor question to [email protected]. Listen to the Car Doctor podcast at johnfpaul.podbean.com.