The tires on my NEIGHBOUR’s ’99 TOYOTA CAMRY have three different tread patterns, tread depths and brands. This two-wheel drive (2WD) car is mechanically happy as long as the same tire size is printed on all the tire sidewalls. However, running a similar hodgepodge of tires on a modern four-wheel drive (4WD) or all-wheel drive (AWD) vehicle might seriously damage the transmission or other costly parts. With many modern 4WD systems, it is very important to replace tires as a set of four and to regularly rotate the tires so they wear evenly.

Tires worn to different tread depths or even new tires of the same size but made by different manufacturers will have slightly different outside diameters and rotate at slightly different speeds. Tires also turn at different speeds every time a car turns a corner so it is hard to understand why a relatively small difference in tire rotations per mile could cause problems for a 4WD system.

to explain further ’86 Ford Bronco. to engage the Bronco’s 4WD you have to move a lever and climb out of the truck to manually lock the front hubs. When turning a corner, all four of the Bronco’s wheels were rotating at different speeds. The front and rear differentials allowed wheels on the same axle to safely rotate at different rates. But, in 4WD the front and rear axles were also connected to each other via the driveshafts and transfer case. During a turn, the wheels on the front axle wanted to move faster than those on the rear, but the front driveshaft could only rotate as fast as the rear driveshaft. The driveshafts prevented the front wheels from rotating as fast as they needed to.

This binding up of the drivetrain did not cause problems in the dirt, rain or snow because the Bronco’s tires could slip slightly on the slick surface. But, driving the Bronco on dry pavement meant the front wheels would hop and extra strain would be put on the drivetrain during turns. Driving the Bronco on dry pavement for a few car lengths was not a problem, but driving it in 4WD on a dry freeway at 60 mph (100 kmh) would have damaged the u-joints, differentials, transfer case or other drivetrain parts.

Jump forward to vehicles of this century. Both cars and trucks are now likely to have 4WD systems that are always engaged. Part-time 4WD like that on the ’86 Bronco is harder to find. The differences between many modern 4WD and AWD systems have become murky, so I will just call them all “4WD.”

Today’s always-on 4WD systems must avoid binding up during cornering like that old ’86 Bronco did, and they must send power to the wheel(s) that still have traction. One common way to do that is to have a transfer case or center coupling filled with clutch packs, viscous fluid or some other components designed to compensate for tire slippage and prevent drivetrain bind-up. The energy of spinning tires or binding parts is often converted to heat. For example, if one tire is spinning then a viscous fluid heats up, thickens and pushes together a clutch that directs the power to another tire that still has traction.

Tires with slightly different diameters moving at different RPMs can damage these 4WD systems by overheating the center coupling. Always-on 4WD systems are designed to drive long distances on dry pavement. However, they are not designed to handle thousands of miles of non-stop tire slippage or thousands of miles of left or right turns, and that is what tires with different diameters simulate.

There may be some modern 4WD systems with center differentials and computer systems designed to safely compensate for tires with larger differences in diameter. 4WD systems vary greatly between manufacturer, model and model year. Following what the specific owner’s manual says about tires is a good idea, but it is always safe to have a set of four matched tires. Anti-lock brake systems (ABS) and limited-slip differentials found on 2WD vehicles also work better with tires of the same diameter. Even my wife’s Tempo would probably handle a little better if all four tires were a little more alike!

examples 4wd engaged 24/7 : 2003 to 2009 toyota 4runner v8, rav4 highlander.
AWD CARS : acura mdx, touareg etc

Informative.

A nice one