Tires & Fuel Economy: Playing by the Numbers
May 2009, TruckingInfo.com - Feature
Spec'ing considerations for tires run the gamut from application to serviceability, and from life cycle costs to anticipated fuel savings. No two fleets will have exactly the same considerations, but at year-end, fuel costs will factor into most spec'ing decisions.
Return on investment must be the main concern when evaluating any fuel-saving measure, including fuel-efficient tires. The device or action must save more money over its useful life (or trade cycle) than it costs.
The argument for spending money to save money was easy to make when fuel was $5 per gallon. At $2 or $3 per gallon, the business case may seem less compelling, but you need to think long-term - or at least as long as your trade cycles.
Is now the time to trade longer tread life for improved fuel economy, for example?
Deeper tread will last longer and yield more miles to removal, but a thinner tread will yield lower rolling resistance right from day one, and might offer fuel savings significant enough to offset the reduction in tread life.
Would low-rolling-resistance duals be a better value in the long run than standard duals, or would switching to wide-base singles be the best bet? Does tire-related fuel economy even matter if you chew through a set of tires within a year in a severe-service application?
There's plenty of evidence, both statistical and anecdotal, showing "fuel efficient" tires can produce significant savings. You could buy based on the evidence, or do the work to calculate ROI for your fleet - but that might prove to be a more complex matter.
According to Michelin Product Marketing Manager Don Baldwin, rolling resistance accounts for about 35 percent of a truck's overall fuel consumption. Mechanical and aerodynamic drag account for most of the rest of the picture, but we can ignore that for this exercise.
Breaking it down further, Goodyear Marketing Communication Manager Tim Miller says the tire tread accounts for about 60 percent of the tire's overall rolling resistance, which leaves the sidewalls and other factors at around 40 percent.
Eliminating two sidewalls and bead areas by switching to wide-base single tires can cut flex-related rolling resistance nearly in half. Switching to a more efficient tread design and perhaps a thinner tread helps, too. Goodyear estimates its Fuel Max tires provide a 4 percent improvement in fuel economy over its standard Unisteel line-haul tires. Continental estimates its HSL and HDL Eco Plus tires can save between 4.7 and 7.2 percent over competitive brands. And Michelin suggests its X One wide-base singles can reduce fuel consumption by between 4 percent and 10 percent compared to a truck quipped with dual tires at the drive and trailer positions.
How to translate all that into a number that means something to your bean counter?
Let's begin with the most basic questions: Can you rely on the tire manufacturer's estimates of fuel savings or rolling resistance reduction, and are the comparisons useful?
Take Goodyear's 4 percent improvement claim for its Fuel Max tires over its Unisteel line-haul tires. At least we have a baseline for comparison, you say. But Goodyear also claims the new fuel-efficient tread design actually produced an 8 percent improvement during SAE testing.
"We saw 8 percent, and we have the data, but we scaled back our claims to reflect actual customer experience," Miller explains. "Things never work out the same in real life."
The way Miller explains it, test results are derived under near-perfect conditions designed to filter out many of the real-life factors that can affect fuel economy. He cautions that cost/benefit analysis conducted solely on SAE-derived reports may not reflect results you'd see in your operation.
SAE fuel economy tests are generally accurate to within a percentage point or two, but in real life, there can be differences of 30 percent or more between your best and worst driver, for example.
Large fleets often do side-by-side comparisons of tire performance. They test and compare apples to apples, and they'll do it on a test track to SAE standards for fast, accurate results. They'll also run the new tires in revenue service for a period of time under fairly controlled conditions, but it may take months to get usable data.
Smaller fleets can take advantage of various fuel savings calculators available through the tire manufacturers, usually on their web sites. Some are restricted to basic inputs like current brand and model, and mileage before removal, which gives a good but basic comparison. Others are more elaborate. Any sales rep will sit down and work through the comparison with you.
Bridgestone Bandag Tire Solutions offers a side-by-side comparison tool, called the Tire Life Cycle Cost Calculator, that allows many more customer inputs, and presumably yields a more accurate comparison. It's not available online, but any Bridgestone sales rep or field engineer can assist you with the exercise.
Guy Walenga, director of commercial products and technologies engineering at Bridgestone Firestone North American Tire, says the tool allows a fleet to compare its current tires against any product offerings from Bridgestone, Goodyear and Michelin. It factors in tire prices, casing values, fuel costs, type of vehicle, load on the vehicle, tread depth at removal, etc., giving a very complete life-cycle cost picture electronically.
"We can provide fuel cost estimates based on customer data input as well as overall life-cycle costs by tire, by axle group, or the overall fleet," he says.
The baseline tires used in the electronic comparison are assigned points on a curve, based on tread depth: new, half-worn, and 2/32, Walenga explains. To get the data, new tires are tested on a drum according to SAE 1269 test procedure to determine rolling resistance at those three points. The computer simulation allows customers to input fleet data, such as removal depth, and the calculator can average out rolling resistance and fuel economy based on test data derived at different tread depths.
"It's as close to a full-life-cycle test as you can get, we think, in a tiny fraction of the time," Walenga says. "We don't even buy the tires we test. We give the testing company a letter and say, go buy and test these tires. They buy the tires off the shelf, so it's a very transparent test, and it yields a very clean database - and we don't always win."
From there, you can gauge the impact of your tire selection on fuel costs and vice versa. Fuel cost is an input variable, so it's easy to compare current fuel costs with projected fuel costs (higher or lower). Rather than going in blind assuming that Tire A is better than Tire B because your pal told you so, the Tire Life Cycle Cost calculator offers the advantage of near-real-world testing to even the smallest fleet.
Trends & Considerations
Tire makers have brought some interesting designs to market that deliver fuel savings, but they aren't without compromise.
Michelin, for example, brought the revolutionary X One wide-base single to market, and it has proved itself hands down.
"In the late '90s, Michelin had hit a wall in terms of technology and compounds advancement," Baldwin says. "The logical next step was to go after an area of the tire where energy losses were affecting fuel efficiency - flexing of the sidewall and in the bead area. We decided to replace two tires in a dual assembly with a single tire, with only two sidewalls working against you rather than four."
The other major tire companies have developed low-rolling-resistance casings, as well as tread compounds and patterns that require less energy to roll, and shallower treads. It's recognized that thick tread squirms and flexes as the contact patch rotates around the tire. This consumes energy, so minimizing that movement saves fuel.
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