Dollars & Sense

Kevin Rutherford

Dollars and Sense

Kevin Rutherford | June 01, 2010

Tires: Less can be more

 

I’m amazed by the large number of owner-operators who look only at miles-to-removal when selecting tires. When diesel fuel was much cheaper, this might have been a good approach, but not today.

To simplify, assume you’re going to choose the proper size and weight rating for your operation and that any brand will have reasonable performance, safety and reliability. That leaves primary considerations of initial cost, expected life and rolling resistance. After running numbers in many scenarios, I’ve found that once fuel tops $2.50 a gallon, the tire with the lowest rolling resistance number that meets your business needs is best.

Tires are rated for rolling resistance. Tires with low RR numbers require less energy than tires with high numbers. Below 50 mph, rolling resistance is the No. 1 factor affecting fuel consumption. Above 50, it’s No. 2, after aerodynamics.

Pay attention to these two factors affecting RR:

TREAD DEPTH. As the tire wears, it becomes more efficient. A tire that’s 30 percent worn becomes 2 percent more fuel-efficient, for instance, and up to 6.5 percent more efficient at 80 percent worn.

Any one tire’s contribution to rolling resistance depends on wheel position. With tandem drive tractors pulling single trailers, tests show about 43 percent of tire fuel economy could be attributed to trailer tires, with tractor tires accounting for the rest. Even if you pull company trailers, your tractor tire purchase decisions still have the largest impact on fuel economy.

Many owner-operators buy more tire than they need. Some drives start life with as much as an inch of tread. Comparing such a tire favorably to a tire that starts off with 24/32s of tread depth may seem like common sense: I’m getting more tire for my money, so it will last longer. But the higher the lug, the more it flexes. The more it flexes, the higher the RR and the faster the tire will wear.

INFLATION. The reason you hear so much about inflation’s impact on fuel consumption boils down to RR. Tests on the effect of inflation pressure over a range from 20 PSI below standard to 20 PSI above found a 2 percent variation in fuel consumption. Pressure is something you can begin maintaining immediately to cut costs, regardless of the type of tires. In addition, proper inflation minimizes irregular wear, yielding longer tire life.

Bottom line, here’s what I recommend in choosing tires and maintaining them:

• Determine what size and style of tire is best suited for your operation.

• Search for the tire that meets those criteria with the lowest RR possible.

• Use a tire calculator to determine the annual fuel savings and compare that to the initial tire cost and life expectancy.

• Buy the tires with the highest return on investment, even if the initial cost is much higher.

• Finally, check inflation regularly. Keep tires balanced and the tractor in alignment. n


How much you can save

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 (97 and 86, respectively) improves fuel economy to 6.52 mpg.

Assume fuel costs $3.15 a gallon and you’re running 125,000 miles annually, and you’ll see that you can save thousands of dollars per year:

Typical (high) rolling resistance tires

Fuel consumption: 125,000 miles / 6.00 mpg = 20,833 gallons

Fuel costs: 20,833 gallons x $3.15 = $65,624


Lowest possible rolling resistance tires

Fuel consumption: 125,000 miles / 6.52 mpg = 19,177 gallons

Fuel costs: 19,177 gallons x $3.15 = $60,408

 

ANNUAL SAVINGS:

$65,624 – $60,408 = $5,216


You can make similar calculations using an RR calculator on my website, LetsTruck.com (under Toolbox, click Tire Calculator).

OverdriveOnline.com strives to maintain an open forum for reader opinions. Click here to read our comment policy.