Tire care tips
by Jim Kerr

Tires. There are hundreds of different tire types on the market today; each designed for a particular purpose or vehicle. Add in all the variations in tire construction: belt types, 4 ply, 4 ply ratings, uni-directional, aspect ratio, speed ratings, rubber compounds, and on and on. The possible combinations boggle the mind, so how can drivers choose new tires? It's difficult. Even the big tire retailers can't keep on top of all the combinations, but there is a wealth of information available to help us.

Some information is found in the vehicle owner's manual. Cars with ABS brakes should use replacement tires with the same size and traction rating as the OEM tires. Truck ABS computers are often reprogrammable for different tire sizes. Both are important for optimum operation of the ABS system.

Tire technology may be changing quickly but the laws of physics haven't. Even the latest run-flat tire designs still depend on traction to keep a vehicle on the road. Those four small tire contact patches rely on proper inflation pressure and sufficient tread depth for traction, so a pressure gauge and a tire tread depth are cheap safety tools.

Wear bars in the tires start showing when there is 2 mm of tread left. These bars show up as solid bands of rubber across the face of the tire. If you have wear bars showing, the tires need replacing. Driving on wet or muddy roads requires more tread depth. New tires start with about 11 to 13 mm of tread but should be replaced when they get down to about 4 mm remaining. Inspect tires on a regular basis, looking for uneven wear caused by improper inflation or incorrect wheel alignment. Catching a problem early can increase tire life dramatically.

In recent years, there has been a large increase in 4x4 vehicles sold with automatic transfer cases. These transfer cases, found in pickups and sport utility vehicles, can be shifted into 2wd, 4wd Hi, 4wd Lo, and Automatic mode. Driving on hard surfaces in 4wd modes causes driveline binding that can wear differentials, transfer cases, and even tires. However, many drivers think nothing of cruising down the highway in Automatic mode. This is when mismatched tires can cause a lot of damage.

Slight variations in tire circumference are normal even for the same tire size, but automatic transfer case damage can occur if the difference is too great. The transfer case computer compares front and rear axle speeds in automatic mode. If there is a difference in speed, such as would occur when tires spin or there is a difference in tire size, then the computer tries to lock the front and rear axles together. This works fine if the tires are actually spinning on loose surfaces, but when caused by different tire circumferences, the transfer case is forced to slip and possibly burn out.

Frequent tire rotation helps keep tire wear even, but the faster the vehicle is driven, the more crucial it is to have tires evenly matched. For example, a difference in front to rear tread depth of only 2/32 inch won't do any damage at low speeds but can start to cause automatic transfer case binding at speeds above 110 kph. A tread difference of 4/32 inch may cause transfer case binding as low as 70 kph. All the tires may need replacing if the difference is too much. This can be an expensive purchase when all you need to do is drive in 2wd mode to prevent damage, but the traction advantages of automatic mode on slippery roads make installing matched tires desirable.

Finally, run-flat tires are increasingly popular as OEM installations, allowing manufacturers to remove spare tires and jacks, and increase cargo volumes. Low tire pressure warning systems in these vehicles notify drivers when tires are underinflated. Some systems monitor ABS wheelspeed sensors to measure rotating rate differences of a low tire, but other systems use pressure sensing transmitters mounted to the rim inside the tires. Care must be used when dismounting these tires, because it is easy to damage an expensive transmitter if the tire is forced against the unit. Information on sensor position is found in the vehicle owner's manual, and let any tire repair personnel know if you have pressure transmitters in the tires to avoid unnecessary repairs.