by Paul Abelson, technical editor
Fuel is in the news. Everyone knows that.
Magazines have been reporting on the problem all year. Some discuss the supply shortfall and the effects of taxation. Some, Land Line included, discuss how changes in driving technique can improve fuel economy. But one question has remained unanswered, mainly because it was unasked all year...'til now. OOIDA Board member Harold Landry wondered about the relationship between tires and fuel mileage. How do various tire specifications improve or reduce fuel economy, and what are the trade-offs between fuel costs and other tire-related costs?
To help answer these questions, I called two colleagues from The Maintenance Council. Peggy Fisher is a fleet tire consultant and writer. She was president of Roadway Tire Co., in charge of all wheel and tire operations for the fleet. At TMC, she rose through various chairs to serve as General Chairman of the organization. Asa Sharp is a retired tire engineer from Goodyear. He made numerous presentations to TMC and contributed to many Recommended Practices. Asa is now a consultant and writer for several fleet magazines. Both Peggy and Asa have earned Silver Spark Plugs, TMC's highest award. With their advice, I know this will be good information.
What fundamental principles apply?
To improve fuel economy, we need to conserve energy. Anything that consumes energy must get that energy from burning fuel. Flexing the tires converts energy to heat and dissipates into the air. Moving a large mass like a tire takes energy as we overcome inertia. If we can reduce flex, we save fuel. If we have less mass to start and stop, we save fuel. If we can reduce aerodynamic turbulence (yes, tires can help), we save fuel. Now, let's look at how we can apply these principles of physics in practical fuel savings.
How does tire size and profile affect fuel economy?
When low profile tires first came out, they were about 4 percent more fuel efficient. Low profile sidewalls are shorter, and therefore stiffer. That reduces flex but only slightly. Smaller tires weigh less than larger tires, so less energy is needed to get them rolling, and also to stop them. The weight advantage only comes into play if you cube-out. If the weight saved on the tire is added to the load, any fuel savings disappear. The added stiffness of low profile tires is no longer measurable, due to improved tire engineering. It is safe to say that your choice between 22.5 or 24.5 wheels, and 11 or metric sizes should be based more on load and speed requirements than on fuel economy. However, since low profile tires on 22.5-inch rims do lower the chassis by about an inch or so, they do reduce under-chassis turbulence slightly, but the difference is barely measurable. It is probably not worth switching to smaller tires, since any gains in fuel economy would be offset by the added cost of re-gearing to keep your engine operating in its most efficient rpm range. Remember, power and economy are affected by transmission, drive axle ratios and tire size (revolution per mile). Change one, and you throw the equation off. With today's new tires, there is virtually no difference in mpg due to tire size.
What is the best tire design for maximum fuel economy?
If fuel economy were the only concern, a simple, straight rib design would be best. The most fuel-efficient tires made are racing "slicks," tires with no tread, as used in NASCAR and CART racing. Those organizations do not race on oval tracks in wet weather. Treads are put in to allow water to escape from under the tire, and to provide traction enhancing edges for the tires' various functions. Slicks are therefore impractical for road use.
Trailer tires use a simple rib design. They are free rolling, except under braking. Steer tires, which use a complex rib pattern, are subject to continual side forces; slight if the truck is rolling straight down the highway, progressively greater as you steer into a turn, and maximum if drive axles are out of alignment. More about alignment later.
The ideal situation is to run purpose-designed tires at all positions: Steer tires with complex ribs on the front axle, drive tires with either full lugs for traction or full-shoulder lug tires for highway use, and simple rib trailer ties. The ideal is not always practical.
Tires flex more when the tread is deeper. Remember that flexing generates heat which hurts fuel economy. As treads wear, traction is diminished, but fuel economy improves. Also, deeper treads last longer. These are all trade-offs to consider.
A typical steer tire has 18/32" of tread depth when new. When it wears to about 5/32", it is usually removed and re-treaded, most often for use on drive axles. New drive tires may have anywhere from 24/32" to 32/32" (one full inch) of tread, depending on design. Hybrid lug tires, with full (solid) shoulder ribs and traction lugs, or blocks of rubber, in the center of the tread, usually have 24/32", while lugs that run the full width of the tread are most often 30/32" or more.
One indicator of how fuel efficient a tire may be is the tread-to-void ratio. The greater the area of the open spaces between tread elements, the easier it is for the tread to flex, so the lower the fuel mileage. The fewer the voids, the better the mileage. Remember, though, the voids define the tread pattern, and patterns are designed for varying conditions: high traction off road, mud and snow, or on-highway operations.
New trailer tires have 12/32" to 14/32" of tread. Being free rolling, they are not subject to as much stress. Often, when drive tires wear to about 12/32" to 14/32", they are not subject to as much stress. Often, when drive tires wear to about 12/32", they are removed and used as trailer tires and new (or retreaded) full tread drive tires are mounted. While this keeps drive tires efficient for traction, it doesn't let the tread wear to its most fuel efficient depth.
Some drivers' carriers will not let them use retreads when carrying hazardous loads like toxins, munitions or explosives. Others carrying high value loads like electronics operate the same way. They run their tires down to about 6/32" in all positions, even through they are allowed to go further. With more tread, they are virtually guaranteed top dollar for their used casings.
Trailer tires contribute most to fuel economy from tires. For the best fuel economy, free rolling, straight ribbed trailer tires should be run as long as possible. Remember, the less tread, the less heat build-up due to flexing. And, there are eight trailer tires, compared with only two steer tires.
Is there any difference between brands of tires?
Yes, but not much, and any advantage one manufacturer may have quickly disappears when the others make modifications to their tires. Any differences are measurable under lab conditions, but almost impossible to notice in real-world operations.
It is, however, generally if unofficially accepted that the "Big 3" - Bridgestone, Goodyear and Michelin - have the most technically advanced tires. Being the biggest, they spend the most on research and development. Other makes of tires, which may not be as advanced, are usually less expensive. There again, you make trade-offs. Depending on how you run, a less expensive tire may save more in purchase price than the fuel you'll save with a more expensive tire. Or it may not.
What about retreads?
The most important things to remember when selecting retreaded tires are:
- Make sure the retreader is reliable. Stick with one of the major retreaders. If possible, inspect their shop for good shop practices, cleanliness and quality control.
- Select the right tread design, just as you would with new tires.
- Make sure the casing on which the tread will be applied has been inspected using the latest technology, and that the casing is sound. Have it inspected even if it is your own.
As far as fuel economy with retreads, a properly retreaded tire on a strong, fuel-efficient casing will give virtually the same miles-per-gallon as a new tire.
How does tire maintenance affect miles per gallon?
There are several things you can do to improve things.
Maintain proper air pressure. Under-inflated tires have increased rolling resistance because they flex more. They flex because you have allowed part of the structure of the tire to be removed. Air supports the casing and gives it stiffness. But over-inflation is as bad as under-inflation. It concentrates wear in the center of the tread, and gives a harsher ride. However, when carrying a heavy load or, and this is especially important, when traveling at speeds above 65 mph, tire pressure should be increased. On a fully loaded (80,000 lb.) rig traveling 75 mph or more, tire pressures should be 115 to 120 psi. You'll get your best fuel mileage at the proper inflation pressure for the load and speed the tire is expected to run. Tire makers have tables available with air pressure adjustments for load and speed.
Make sure wheels are aligned. In a recent study done at several typical truckstops, most tractors not only had one or more tires under-inflated 10 percent or more, most also had drive tandems out of line. When set to the average misalignment found in the study, test trucks, with drive axle alignment as the only variable, got about 2.2 percent poorer fuel mileage. That was based on real-world averages. If your truck is misaligned more than the average, you will have even lower mpg.
Other fuel saving tips were offered in previous issues of Land Line. We hope this answers some questions you may have had concerning tires' effect on fuel economy. Once again, thanks to Peggy Fisher and Asa Sharp for their help researching this information.
Paul Abelson is Land Line's technical editor and freelances from his office in Lisle, IL.