Passenger tires, typically purchased for vehicles that will not see such severe service, are not recommended for police duty. "You are going to lose the speed rating, and if the vehicles were involved in a pursuit, the tires could introduce unfavorable handling characteristics," says Miller, who spent nine years as a police officer and small-town police chief. "Speed-rated tires used on police vehicles have more in common with high-performance tires on sports cars than tires used on a civilian version of the same car used by police."
Tires rated W to Z pass muster as pursuit-rated tires, with "Z" being the highest available.
If a tire races along at 150+ mph at anything less than a Z rating, it would likely disintegrate, Longobart stresses.
The U.S. Department of Transportation also requires tire manufacturers to grade passenger vehicle tires on three performance characteristics: treadwear, traction and temperature resistance. These ratings also factor into police tire selection.
Treadwear. This rating refers to the general hardness or wearability of a tire's rubber compounds, which directly translates into how long and how well the tire will hold up. A tire with a lower treadwear ranking may only be rated to last 40,000 to 50,000 miles, while one with a higher treadwear score may withstand 60,000 to 80,000 miles. For instance, says Alley, Goodyear rates its Eagle RSA17 at 260, which means it wears about 2.6 times as well as a tire ranked at 100. "From my days in law enforcement, I can tell you we certainly didn't get that kind of mileage out of our tires," Miller admits. "But even so, treadwear is a critical consideration in an agency's purchasing decision. Treadwear ratings directly impact how soon you may need to replace the tire."
Temperature. Tires are made of rubber, and heat is its natural enemy. Any time a tire rolls down the road it generates friction which produces heat. The faster one drives the greater the turning forces exerted on a tire and the more heat produced. Manufacturers design tires to expand and dissipate this heat build up under severe operating conditions to ensure they maintain integrity. This is what temperature grade speaks to. Temperature grades, from highest to lowest, are A, B and C. These rankings represent the tire's resistance to heat generation. The Goodyear Eagle RSA17 puts its pursuit-rated tire at an A, says Alley, who's been with the San Diego PD for eight years.
Traction: There is both wet and dry traction. Manufacturers design tires with specific characteristics and performance metrics in mind. In some cases, the manufacturer tries to hit that sweet spot offering a blend of both wet and dry performance. Traction ratings, from highest to lowest, are A, B and C. They represent the tire's ability to stop on wet pavement. Again, Alley says Goodyear puts its Eagle RSA17 at an A rating.
"We use Goodyear because it's the manufacturer's original equipment," he says. "Also, if we have to change one tire, we could not put on a different tire make or model. If we did that, we'd have to replace all five tires, and that would be cost prohibitive."
Most departments employ a competitive bid process to purchase tires. Alley notes his agency bids competitively every five years. But he adds they spec a single tire brand.
"We've found that if we stay with one manufacturer it reduces our costs overall," he says.
Keep it up
Ultimately, a tire's ratings won't mean much if a department fails to maintain its tires.
Over- or under-inflated tires signify a significant problem on the road, says Alley. Under inflation ranks as the leading cause of tire damage and failure, and can increase fuel costs by 3 to 5 cents a gallon - something today's cash-strapped agencies can ill afford. Over inflation stimulates heat build up and boosts the risk of high speed blow outs. Out-of-balance tires trigger uneven wear and vibration, creating unstable conditions at high speeds and shortening tire life.
"Under inflation is one of our biggest challenges," Alley admits. "Tires naturally lose air over time."
For this reason, motor technicians in this California department inspect tires daily before each shift. While manufacturers recommend vehicles be brought in every 5,000 to 7,000 miles for maintenance, San Diego pulls its patrol cars for routine repairs every 3,000 miles. At this time, technicians remove the tires and inspect them for damage, set air pressure to 35 psi, then balance and rotate them. They replace any tires at 4/32-inch or less tread. Technicians plug holes in tires only once, after that the tire must be changed out.