What You Don't Know About Tires Can Hurt Fuel Economy
Aerodynamics, engines, speed, idling, vehicle weight, these are all things that come to top of mind when considering improving fuel economy on heavy trucks. But there's another component on the truck that plays a critical role in fuel economy that is sometimes overlooked when pondering the fuel economy equation. One of the easiest ways to control big rig fuel consumption is by properly selecting and maintaining tires. Consider these facts:
- Aerodynamic drag accounts for approximately 40% of fuel consumption.
- Rolling resistance accounts for approximately 25% of fuel consumption.
- Tires make up 35% of the energy consumed by the vehicle. In its publication "Secrets of Better Fuel Economy," Cummins shares tire fuel economy facts every trucker should be aware of:
- Worn tires provide better fuel economy than new tires, up to 7% better fuel economy.
- Used lug drive tires can get up to 0.4 mpg better than new lug tires.
- Ribbed tires on the drive axles provide 24% better fuel economy than lugged tires.
- Every 10 psi that a truck's tires are underinflated reduces fuel economy by 1%.
- The break-in period for tires is between 35,000 and 50,000 miles.
- Tires make biggest difference in mpg below around 50 mph; aerodynamics is the most important factor over around 50 mph. Rolling resistance is the second largest consumer of power on a truck, so the type of tire and tread design have a considerable effect on fuel economy and performance. Fuel-efficient tires can produce a 3% reduction in rolling resistance that translates into a 1% fuel savings or an increase of .05 mpg.
Rolling resistance results from the internal friction of a tire as it flexes during motion. Tires flex more at higher speeds. This leads to more friction, higher tire temperatures and reduced fuel economy. Increased weight causes increased flexing of the tires. So properly selecting tires rated for the loads carried is critical. New developments tire design and construction have had considerable impact on fuel economy. Radial tires are one of the most significant improvements in tire technology. In early tests, Bridgestone found that converting from bias-ply tires to radials improved fuel economy more than 10%. This represents about a 30-40% decrease in rolling resistance. Much of the rolling resistance of a tire, about 35-50%, comes from the tire tread. For that reason, many manufacturers have focused on fuel-efficient tread compounding. According to Bridgestone, some compounds, especially those incorporating silica, or using special formulas that combine natural and engineered-structure synthetic rubber, can reduce tire rolling resistance significantly. Tread depth also has a significant effect on tire fuel economy. Bridgestone tests show that as a tread wears, the fuel efficiency of a tire usually increases. With less tread, they weigh less, and because the shallower tread is less subject to energy-wasting squirm. A 7/32 tread wear represents a10% reduction in rolling resistance (5% better mpg) compared to a new tire. A worn tire is about 7% more fuel efficient than a new tire. Tread design changes fuel economy too. Usually, shallow treads are more fuel-efficient than deep ones. And, rib designs tend to be more fuel-efficient than lug or block designs. A new lugged tire is less fuel-efficient than a new ribbed tire by about 6%. Rib tires at all wheel positions will provide greatest fuel efficiency, according to Cummins.
Casing design can also impact fuel economy. Casings (including belts) contribute about 50- 65% of tire rolling resistance. Tires that run hotter consume more fuel and a majority of this heat is generated by sidewall flex. New and improved casing designs minimize sidewall flex and reduce running temperatures. Improved rolling resistance can be achieved even more by using wide-based tires on drive and trailer axles. Closed track tire testing has shown as much as 4% mpg improvement over conventional dual rib tires on drive and trailer axle positions. As important as proper tire selection is proper tire inflation, according to EPA SmartWay. When not properly inflated, tires flex more under load. This produces heat and increases rolling resistance, which wastes fuel. Truck tires inflated 10 psi below recommended air pressure levels can reduce truck fuel economy between 0.5%and 1%. Since the bulk of the load is carried in the trailer, a 10-psi under-inflation in a trailer tire may have nearly twice the impact on truck fuel economy as the same amount of under-inflation in a drive tire. Heat and stress from improper inflation softens and deflects tire components, causing faster and more uneven wear, which shortens the life of the tire. Underinflated tires have more frequent punctures, increasing the risk of tire failures that could lead to costly road service and loss of revenue. Despite the importance, EPA says a recent survey of combination trucks found that less than half the tires surveyed were within 5% of the recommended inflation pressure. Another industry survey indicates only 8% of truck drivers check tire pressure with a tire gauge before each trip. One reason fleets may find it difficult to keep tires properly inflated is because truck tires can lose up to 2 psi each month, even if the rim seal and valve stems are tight. A fleet may not be able to inspect or monitor its' trailer tires regularly due to the fact that extended periods of time are spent away from service yards and because trailers are interchangeable. This places greater responsibility for checking tire pressure onto drivers. One solution to the inflation problem is spec'ing automatic tire inflation (ATI) systems that monitor and continually adjust the level of pressurized air in tires, maintaining proper tire inflation automatically even while the truck is in motion, eliminating the need to check tire pressure manually. Automated tire inflation systems can be costly: Installing an ATI system on a truck's drive axles and trailer costs up to $800. However, for a typical long-haul combination truck, annual fuel savings could reach 100 gallons, saving $380 in fuel costs, according to EPA. Annual tire maintenance costs can also decrease, so cost of installing an ATI system in a long-haul truck is generally recouped in just over two years through fuel and maintenance cost savings. EPA's SmartWay Technology program establishes credible performance criteria and reviews test data to ensure that equipment and technologies will help fleets improve fuel efficiency. The program has determined that certain tire models can reduce fuel use by 3% or more, relative to the best selling new tires for line haul class 8 tractor-trailers.