Spec the Perfect Tires
Rolling resistance is a major factor, even at low speeds, says Giti’s Estupinan. “Just as with air resistance, the actual amount of rolling resistance is influenced by many factors, including load, speed, inflation pressure, tread pattern, tire design and construction.”
Walenga says tires designed for fuel economy that show a 2 percent improvement in highway fuel economy may show only a 1 percent improvement in cities. “The problem is that there is so much fuel used in acceleration and braking that rolling resistance becomes a smaller part of the picture.”
Michelin’s X One wide singles save 220 pounds per axle, Baldwin says. Consequently, they have fuel economy advantages in stop-and-go driving, where the vehicle and its cargo must repeatedly accelerate and then lose energy upon braking.
Tire size and weight rating
Size and the related weight rating are the most critical variables in tire spec’ing. “It’s always smart to stay with what was specified originally,” Walenga says of the size. “You’ll see axle load listed on a placard on the vehicle. It may even list the specific tire size.”
This is a critical point in avoiding citations and out-of-service orders, too. Whether tires on an axle are rated to carry the full axle rating is a part of the Commercial Vehicle Safety Alliance inspection criteria. “You’ll be fined as an individual and as a company if the tires are not sized to the load,” Baldwin says.
Walenga and Miller say you can easily check to make sure a size will do the job. Both refer to the data books that tire manufacturers publish, and Walenga adds you can go to websites of organizations such as the Tire & Rim Association (www.us-tra.org) to get a standard list of weight capacities and pressures. Consulting such a chart will help you determine the correct pressure to use for higher load range tires, too.
Also, keep in mind that changing the diameter of drive tires will alter your effective gear ratio, which might cause you to cruise outside the engine’s sweet spot.
“Nearly every tire choice is something of a compromise,” Miller says. “In general, steer tires are usually rib-type tires with relatively medium tread depth. Drive tires, when traction on snow or mud might be a factor, tend to be more ‘aggressive’ with cross grooves (lugs) and deeper tread depths. If a fleet of over-the-road trucks never leaves Florida, the fleet would benefit (in both wear rate and fuel economy) from use of rib tires on every wheel position.”
But any vehicle that needs extra driving traction in dirt, mud, slush or snow will be better off with lug-type drive tires for traction. “Tires for single-drive-axle tractors should be more aggressive than tires for tandem tractors, because there are only half as many drive tires,” Miller says.
Walenga says, “If running well over 10 percent of the time off-road, even with mostly highway operation, you need an on-off road tire with a tread that’s much more cut and chip resistant.”
When a truck runs off-highway, “chipping and chunking of the tread are a possibility,” Miller says. “This possibility is reduced with the use of tire tread compounds that resist such damage.”
On the trailer, particularly in over-the-road operations, “run rib-type tires similar to steer tires but with even less tread depth,” Miller says. “Too much tread on a trailer tire seems to result in the development of irregular tread wear that tends to shorten the overall life of the tire.”