By Paul Abelson
senior technical editor
When fuel prices started rising, operators started paying attention to something tire engineers have known for years. Tires affect fuel economy.
Energy from fuel overcomes mechanical resistance in the engine and drivetrain. Aerodynamic drag consumes the greatest share of energy at speeds above 50 to 60 mph (depending on truck configuration). It increases geometrically as speed increases.
Not all driving is at high speeds, and at lower speeds the greatest consumer of energy is rolling resistance, the flexing of tires during driving. On average, rolling resistance accounts for about 35 percent of fuel consumption. Making tires perform more efficiently is one of the best ways to improve fuel mileage.
Tire design and inflation pressure affect fuel mileage. Air is what supports the truck and load, and the tire contains the air. With too much air, the tire becomes stiff and will not function as designed. And an overinflated tire is more susceptible to damage.
An underinflated tire is soft and compliant, but as pressure decreases and flexing increases, internal friction increasingly heats the tire. Excess heat is a primary cause of thrown treads because it weakens internal structure and destroys the bond between steel cords and rubber.
When tires are underinflated, they flex more than they’re supposed to. That makes them heat up too much. That heating wastes fuel, costing you fuel mileage.
Keeping tires inflated properly maintains fuel economy. Tire makers have worked on design and engineering to improve fuel economy in ways we see and in some we don’t.
Perhaps the most obvious and dramatic change was the introduction of wide-base singles to replace a set of duals. Michelin X One and Bridgestone Greatec tires save fuel in part by removing one-half the number of sidewalls at drive and trailer-tire axle ends. Sidewalls contribute significant rolling resistance, so turning an 18-wheeler into a 10-wheeler can improve mpg 6 to 8 percent by eliminating sidewalls.
Less obvious are the internal improvements in materials and design. Higher strength steels yield stronger, lighter belts. Polymer chemistry has improved properties of tire rubber, increasing abrasion and impact resistance and lowering internal friction.
These improvements are not confined to wide-base singles. Goodyear, Yokohama, Hankook and Continental make fuel-efficient steer and dual tires that have been approved for the EPA’s SmartWay Program. Michelin has several models of drive and trailer duals that also qualify.
SmartWay criteria are based on SAE/TMC fuel economy testing. When used in steer, drive and trailer positions, tires accepted by SmartWay must demonstrate a reduction of nitrogen oxide emission and an estimated fuel savings of 3 percent or better compared with the best-selling new tires for line haul trucks when used in all positions.
Fuel savings from tires are not confined to new tires. Properly selected low-rolling-resistance tires with fuel-efficient treads will yield comparable retreads, according to Harvey Brodsky of the Tire Repair and Retread Information Bureau. The group is currently working with EPA to get retreads SmartWay approved and is reported to be close to finalizing the program. LL