DENVER (March 14, 2018) – Think fast: Can you name Colorado's most popular car? If you said the Subaru Outback, you'd be wrong – but you wouldn't be far off. The Outback comes in at number three in Colorado vehicle registrations, just behind the Ford F-Series and the Dodge RAM, per data from IHS Markit.
Now, guess what those cars have in common. You got it: They look super cool when tackling Colorado's mountain roads. Know why? Each is equipped with either a four-wheel-drive (4WD) or all-wheel drive (AWD) system to maintain traction in treacherous and even off-road conditions. In Colorado, our cars are as active as we are – look no further than the dozens of 4WD or AWD vehicles on any given Colorado road, pass, or trail.
If you're one of the thousands of Coloradans who own a vehicle equipped with either of these drive systems, though, take good care: Both are extremely sensitive to variations in tire tread. That means, as these motorists replace a flat or gear up for spring by swapping out their tires, they must ensure that all four tires are of the same brand, model, size, and have the same degree of tread wear – or risk expensive drivetrain damage. Some systems are so sensitive that more than ¼" of tire circumference variation can cause major drivetrain component failure.
Per research from the AAA Automotive Engineering and Repair division, Subarus are especially sensitive to tire-tread variations. Assuming all four tires are the same brand, model, and size, the tread depth between them may not vary more than 2/32". Failure to adhere to these requirements makes it a matter of when, not if, the transmission or another drivetrain component will be damaged. Audis and Porsches similarly experience serious drivetrain problems when tire diameters vary too much.
Luckily, ensuring consistent tire tread is simple – and can save drivers money when compared to the cost of repairing or replacing a drivetrain. Motorists should have their tire pressure checked at least once a month and should ensure pressure on each tire matches manufacturer specifications. Tires should be rotated per the manufacturer's recommendations to equalize wear. And, when installing anything other than a complete set of original-size new tires on a 4WD or AWD vehicle – if you're replacing just one tire, for example – check your owner's manual for treadwear, diameter, or circumference variation specifications.
Install a matching set of four new tires. This is often the best option when the other tires on the vehicle are somewhat worn. The less tread that remains, the more sense it makes to install all new tires. However, if a car owner needs to save money, other options can be considered.
Install two new tires that are an exact match for the others on the vehicle. This option applies primarily to part-time 4WD or on-demand AWD systems, provided the circumference variation of the new tires is within specifications. The two new tires should always be installed on the rear axle to maintain vehicle stability in low traction conditions.
Install one new tire that is an exact match for the others on the cars. This is a good option when the other three tires have minimal wear and the circumference variation of the new tire is within specifications. If the variation is too great, the new tire can be shaved using a tire truing machine to match the average circumference of the other tires.
Install one used tire that is an exact match for the others on the car. This option depends on being able to locate the exact make and model of tire with the same degree of treadwear as the other tires on the vehicle. Subaru and Audi dealers, which offer many AWD models, sometimes keep good used tires on hand for times when a single tire needs to be replaced. Used tires are also available online and at local tire resellers, but locating and purchasing an appropriate tire through these channels can be time-consuming, and tire quality is a potential issue. No matter where you buy, make sure to check the DOT date code and be aware that many manufacturers caution against using tires more than six years old.
"If you're anything like most drivers, you don't spend a whole lot of time thinking about your tire tread," said AAA Colorado spokesman Skyler McKinley. "But what you may not know is even tiny variations in tread between tires can cause devastating damage to your vehicle – especially if, like many Coloradans, you have a four-wheel-drive or all-wheel-drive system. You can avoid steep repair costs with two key practices: Maintain vehicle tires properly, and choose replacement tires thoughtfully."