Weight Savings & Fuel Economy - Tire Review Magazine

# Weight Savings & Fuel Economy

While cost savings will vary depending on vocation, application and operating conditions, states a Kenworth White Paper published a few years ago, weight savings can improve the lifecycle cost of a vehicle in two respects: increased payload capacity and better fuel economy.

“Lighter weight reduces the amount of horsepower required to overcome mechanical resistance of the drivetrain and tires while driving,” the paper said. “Based on \$2.50 per gallon fuel and 125,000 miles annually, 1,000 lbs. of weight savings on a single vehicle can generate up to \$250 in annual fuel savings. For a 500 truck fleet, lightweight specs could lower fuel costs by nearly \$150,000 annually.”

While fuel costs today are much higher, the example Kenworth provides is still valid. To determine how weight reduction could lower fuel costs, the OEM suggests using the following formula:

Fuel Savings = Weight Savings x Tire Rolling Coefficient x BSFC x Miles/Year x Fuel Cost/Gallon x Number of Trucks divided by Conversion Constant.

As defined by the OEM, the standard Tire Rolling Coefficient for radials is 0.006; BSFC (Brake Specific Fuel Consumption) measured in lb./HP – hr. is 0.35; Conversion Constant (for English units such as pounds, miles, horsepower) is 2,700. Using the example above:

1,000 lbs. x 0.006 x 0.35 x 125,000 x \$1.50 x 500/2,700 = \$72,917.

One weight and fuel saving option that is growing in popularity are super wide single tires. What kind of fuel economy that wide single tires can provide? A lot…if only because a great deal of the fuel consumption attributable to tires is the result of sidewalls flexing with each revolution. With super wide singles, there are half as many sidewalls flexing, something Bridgestone Americas estimates could result in savings of 2% to 5% on fuel.

Bridgestone Greatec super wide radials, for example, are available in the 445/50R22.5 size, equivalent in overall diameter to regular low-profile radials for 22.5-inch wheels. Depending on the types of tires and wheels converted, Greatec radials could reduce overall vehicle weight by 600 to 1,200 or more pounds.

In addition to directly converting vehicle weight into revenue-paying load, according to the manufacturer, Greatec radials could also provide fuel economy advantages in comparison to conventional, non-fuel-efficient dual tire assemblies. Actual fuel savings will vary, depending on vehicle configuration, how many axles are converted, what tires were used previously and other factors.

Michelin also touts the fuel savings advantages of its super wide singles. The manufacturer sells its line of X One radials, which it says are designed to provide a combined weight savings in a range from 740 to 1,370 lbs. per tractor-trailer combination, or close to 200 lbs. per axle. Using 6 mpg, 100,000 miles per year and fuel costs of \$2 per gallon for comparison purposes, the company says X One tires can deliver fuel savings of at least 4%.

Some of the weight-savings associated with super wide tires is from the concurrent use of aluminum wheels. Alcoa aluminum 14-inch wide wheels in place of duals, according to the company, can save 318 lbs. per axle and up to 1,272 lbs. per combination vehicle.

Alcoa aluminum 14-in. wide base wheels weigh 71 lbs. each and are load rated up to 12,800 lbs. But weight isn’t the only reason to choose a wide base configuration, the company states. Benefits include fuel efficiency because wide base wheel and tire assemblies have half as many flexing sidewalls as a set of duals. Less flexing leads to greater fuel efficiency, the company adds, with some fleets experiencing fuel savings of three to seven percent.

Tim Miller of Goodyear does add a note of caution about super wide tires. “From a weight-savings standpoint, converting from conventional tires on steel wheels to wide-base tires on aluminum rims can save over 1,000 lbs. in total vehicle weight,” he said. “It is true that wide singles may have less rolling resistance than many conventional tires but, with recent advances in fuel efficient tire designs, the difference can be nearly impossible to measure in real-world situations.”

As for calculating savings potential in cost-per-mile with wide base singles, Miller advises to factor in weight, fuel economy, downtime, retreadability and vehicle resale. Spec’ing for fuel economy by reducing vehicle weight, it seems, requires a measured approach.

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