We continually receive inquires related to four-wheel drive (4WD) and all-wheel drive (AWD) tire replacement. A frequent question is “Do I really need to replace all four tires on my vehicle?" While the short answer is often yes, we'd like to help explain the importance of this response.
To properly address this concern, it's important to understand the basics of the drive system. 4WD and AWD vehicles allocate the engine's horsepower to each of the four tires as needed (via a differential) – enabling the wheels to turn at different speeds. “The best system will send exactly the right amount of torque to each wheel, which is the maximum torque that won't cause that tire to slip." (How Four-Wheel Drive Works). This is particularly helpful on loose gravel, sand, slippery roads, and other adverse conditions.
Most differential systems are designed to allow short-term differences in wheel speeds when the vehicle turns a corner or spins a tire. If you use mismatched tires (tires of different size, tread design or tread depth), you may be forcing the differentials to operate at all times. This will lead to excessive heat and unnecessary wear on the differential, and may cause early failure.
In fact, studies have shown that differences of just 2/32 of an inch in tire tread between tires on the same vehicle can impact vehicle performance. For example, if you were to replace just two tires on your AWD or 4WD vehicle, you would have a slightly larger diameter on the new tires, due to deeper tread. A small difference in the front and back tire diameter means the two axles are always turning at different speeds, even if the car is going straight down a highway. This is contrary to how the vehicle was designed to operate and will likely lead to early malfunction.
In simplest terms, when tires have a different circumference, they have a different number of revolutions per mile. The smaller the tire (less tread), the more revolutions per mile required. The larger the tire (more tread), the less revolutions per mile. When front and rear axles rotate at different speeds, it creates added strain on the vehicle's driveline. The greater the difference in the tires' circumferences the greater the resulting strain.
For this reason, it's essential that 4WD and AWD vehicles use tires that are matched in size. Tire diameter variations can be caused by
accidentally using different sized tires, tires worn to different tread depths, tires with different tread designs, tires made by different manufacturers, or even different inflation pressures.
If you own a 4WD or AWD vehicle, you should be aware of the importance of maintaining four matching tires. As always, routine maintenance is your best solution to minimizing potential issues with your vehicle. This includes tire rotations, alignments, maintaining proper air pressure, and monitoring tread depth. “Nearly every U.S. state has a minimum tread requirement for all passenger car tires of 2/32 inch. Driving on tires with tread lower than this minimum depth is not safe or legal." (eHow).
We hope this article helps answer some of your questions or concerns on this topic. We realize this can be confusing and frustrating for AWD and 4WD vehicle owners, but we are here to help answer your questions. Safety, and proper vehicle performance and maintenance are top priorities to us. As always your comments are welcome. As a policy, Dunn Tire will follow the tire replacement recommendations in each vehicle's owner manual. We don't create these guidelines, but they are in place for a reason. The top priorities are vehicle safety and optimal performance. We do not cut corners nor attempt to mislead customers.
Below are different companies' comments from owner's manuals or technical bulletins.
Nitto Tires technical bulletin:
Attention! – Matching Tires on Four-Wheel Drive and All Wheel Drive Vehicles:
4WD and AWD vehicles require special attention to insure that all four tires are closely matched in diameter to avoid strain and possible damage to the vehicle's differentials and/or viscous couplings. Tire inflation pressure also affects the tire's rolling circumference and should be matched according to the vehicle manufacturer's recommendations. Always check the vehicle manufacturer's recommendations prior to installing new tires.
In some cases, the vehicle manufacturer may specifically advise against replacing less than all four tires. Always check and follow the recommendations in the vehicle owner's manual. For 4WD and AWD vehicles, even small differences in outside diameter may cause drive-train damage or mechanical malfunction.
Do I have to replace all four tires on my AWD Subaru?
All of the tires on your AWD Subaru must be within 1/4 of an inch of rolling circumference (part that touches the road). This is because of our All Wheel Drive System. See page excerpt for a 2010 Subaru Forester owner's manual here. You can look up your specific Subaru owners manual here.
Proper rotation of the tires at the appropriate service intervals will increase the life expectancy of your tires. This will also ensure that all four tires stay relatively equal in their tire tread wear. When vehicles are serviced, tires should be routinely checked to ensure that the alignment and tires are in good working condition.
Depending on the Tread on the other tires you may need to replace all 4 tires. You can only have about a 3/32 tread difference between tires on any AWD or 4×4. The reason is with the different tread depth the tires will rotate at different rates and put un-wanted stress on the AWD drive train that can cause damage to it.
Not sure if your auto is AWD or 4WD? Check your owner's manual and read up here on the differences.