A hidden cost of AWD
All-wheel-drive vehicles offer drivers enhanced balance, stability, traction and handling. With the grip of four driving wheels, these vehicles can more easily climb snow-packed hills and effectively plow through the bumper-deep drifts.
But there is no free lunch. The all-wheel-drive option costs extra, is less fuel efficient than a two-wheel drive counterpart, and utilizes additional components that are subject to wear. On top of that, there is another “hidden” cost facing owners, having to do with tire requirements.
Again, all-wheel-drive and four-wheel-drive vehicles have an advantage on slippery road surfaces because they can distribute power to all four wheels. Systems differ, but most divide power by reducing it from wheels that are spinning and adding it to wheels that are not. Therefore, all wheels are mechanically connected, which requires a system of viscous clutches or differential couplings to allow one wheel to turn faster when spinning or turning a corner.
So, especially with computer-enhanced transfer cases offering traction and stability control, all-wheel-drive systems require closely matched tires in all four positions. This means that if you need to prematurely replace a tire due to damage, you’ll likely need four.
Dimensions of tires with the same size designations vary among manufacturers. So, if the replacement tire is not the same brand and model as the others, or if the other three have a degree of wear, the dissimilar-circumference replacement tire will cause the viscous coupling to “slip” at all times, causing undo heat and wear.
A new tire of the exact same brand will be also be a questionable replacement if, for example, the new one has 10/32-inch tread depth and the used three have only 7/32-inch remaining tread. That translates to about one-half of an inch of extra circumference on a same-brand new tire, meaning that it would spin slower than the rest — by several less revolutions-per-mile.
Maintaining the proper tire pressure is also crucial on all-wheel-drive autos, but using matched tires is a must to reduce drive train strain. Industry standards say “matched” tires must be of the same brand, design, and tread depth. Tires failing those definitions may lead to premature failure of drive train components.
Computer-controlled traction systems with electronic speed sensors may have immediate adverse effects borne of tire-size mismatch such as shuttering, binding, or total system disablement from the tire-size variance.
Regular tire rotation is critical for all-wheel-drive and four-wheel-drive vehicles — every 5000 miles is recommended. Each tire has a different role in cornering, braking, and accelerating, which leads to uneven wear. Tire rotation is required to even out tread wear and maintain equivalent tread depth (hence circumference) for all four tires.
Vehicle manufacturers have varying recommendations for their all-wheel-drive products, ranging from suggesting exact circumference to two or three 32nds difference. Regardless of anyone’s recommendation, the least stress is applied to this style of drive train when all four tires are the same type and have equal tread depth.
But don’t let the tire requirements dissuade you from buying an all-wheel-drive vehicle. They prove their worth during harsh winters and on any slippery streets. If you maintain proper air pressure and rotate them regularly, your tires should wear out equally. If just one is damaged however, be aware of the “hidden” cost of buying a whole set, or finding an exact match to the remaining three.
Readers may contact Bill Love via e-mail at precisiondriving@spokesman.com.
