Tire Spec’ing

Overdrive Staff | August 07, 2011

Keep your No. 2 variable expense to a minimum by spec’ing right.

Just as a truck with improperly geared rears will incur extra costs for its owner, so, too, will a truck with the wrong tires. There are many more factors than size that account for having the right tire for a specific truck in a specific application. The variables to consider when spec’ing tires are many. Among them are the size and number of plies and related weight rating, tread design, tread depth, tread compound and wheel position.

All play a role in proper tire spec’ing, and smart spec’ing can save costs in two ways: lowering fuel cost per mile, and lowering tire cost per mile. Attributes that lower one cost can have the opposite effect on the other cost, so spec’ing needs to be done carefully. That’s easier said than done in an era where specialization abounds. Tire makers are developing products that are focused to precisely meet specific service options.

Just as a truck with improperly geared rears will incur extra costs for its owner, so, too, will a truck with the wrong tires. There are many more factors than size that account for having the right tire for a specific truck in a specific application. The variables to consider when spec’ing tires are many. Among them are the size and number of plies and related weight rating, tread design, tread depth, tread compound and wheel position.

All play a role in proper tire spec’ing, and smart spec’ing can save costs in two ways: lowering fuel cost per mile, and lowering tire cost per mile. Attributes that lower one cost can have the opposite effect on the other cost, so spec’ing needs to be done carefully. That’s easier said than done in an era where specialization abounds. Tire makers are developing products that are focused to precisely meet specific service options.

Since longer tire life means lower costs, the effort put into careful spec’ing will pay off well. The four main tire applications are long haul, regional, on/off-road and urban. The different applications mean very different tire lives, from as little as 20,000 miles for urban to more than 200,000 miles for a long-haul team. Specific steer, drive and trailer tires are available for each application.

If you’re spec’ing a new truck, you can pick any tire size suitable to your application. A new truck’s drivetrain and engine controls are set according to the spec’d tire size.

Consequently, replacement tires of different specs can require drivetrain and engine adjustments. Changing tire sizes also might cause clearance issues.

More isn’t always better. Rather than last longer, tires with deeper tread might wear faster, burn more fuel and make driving harder if they’re wrongly applied. And the traditional miles-until-removal measure of tire performance more often now plays second fiddle in long haul to picking tires based on fuel economy. Some regional fleets are going to long-haul steers for low rolling resistance rather than tires with more scrub resistance designed for regional operation, for instance. The tires may wear faster, but they save enough in fuel to more than cover the extra tire replacement costs.

Consider this common tractor-only scenario: The steers have a 125 rolling-resistance rating, the drives are at 137, and the truck gets 6 mpg. Replacing steers and drives with lowest possible alternatives for RR rating (97 and 86) improves fuel economy to 6.52 mpg.

TYPICAL (HIGH) ROLLING RESISTANCE TIRES

Fuel consumption:

125,000 miles/6 mpg = 20,833 gallons

Fuel cost:

20,833 gallons x $4 = $83,332

LOWEST POSSIBLE ROLLING RESISTANCE TIRES

Fuel consumption:

125,000 miles/6.52 mpg = 19,177 gallons

Fuel cost:

19,177 gallons x $4 = $76,708

ANNUAL SAVINGS:

$83,332 – $76,708 = $6,624

In addition to the basic spec areas involving tire position, tire life and fuel efficiency, here are other factors to consider when choosing tires:

SHOP FOR THE BEST VALUE. It’s tempting to shop primarily by sticker price, but cost per mile and retreading potential are more important considerations. Making an informed choice means keeping written and dated records of purchase, fuel mileage and tread depth, then comparing records of models you’ve used. Also, consider warranty in any value calculation.