Why Fuel Mileage Drops In The Winter
Fleets can expect as much as a 17% decrease in mpg when Old Man Winter hits hard.
Jim Winsor
Executive Editor
It's October already and temperatures in the North Country will be dropping for the next five months or so - as will diesel fuel economy for many fleets that operate in cold areas. Mother Nature can indeed have an impact on truck and engine efficiency.
At last month's meeting of the Technology and Maintenance Council (TMC), one technical session entitled, "Why Does Fuel Economy Go South Each Winter," examined the phenomena and the reasons for a drop in fuel efficiency.
The most obvious culprit is the driver, said Roger Duff, director of equipment and maintenance for Alvan Motor Freight, a regional LTL carrier headquartered in Kalamazoo, Mich.
Duff says he's been through a lot of Michigan winters, and he claims engine idling is what is sucking up the fuel.
"The amount of idling goes up pretty much in the ratio that the temperature drops," he said.
No one likes to climb into an ice-cold truck, he continued. And there's a tendency to idle for longer periods when the temperatures hit zero and below. Then there's concerns about cold-morning starts when it's really cold. There's also the issue of winter grade diesel, a blend of No. 2 and No. 1 diesel. But more on this later.
Asa Sharp, a retired Goodyear truck tire executive, said tire tread, alignment and inflation all impact fuel mileage. Misalignment, especially dog-legged trailers, cause tire scrubbing, which increases fuel consumption.
Cold tires from overnight parking take an hour to warm up and until they do, they have higher rolling resistance, which also contributes to fuel usage. New deep-tread tires also have higher rolling resistance, but safety and the need for maximum traction often dictates mounting new tires for winter.
Lubrizol's Allan Ross said axle lubricant viscosity can contribute a 1.5% to 2% fuel consumption increase in trucks using traditional gear lubricants. It takes upwards of an hour for axle and transmission lubes to thin out during cold spells. He said the use of synthetic lubes in axles/transmissions often pay for themselves in fuel savings in the Cold Belt.
Jim Ross, a research engineer from the NASA Ames Research Center, U.S. Dept. of Energy, said research on the effects of airflow on fuel economy shows that at 70 mph, it takes 65% of a vehicle's horsepower to overcome aerodynamic drag.
That's one reason why high speeds bring higher fuel bills. Aerodynamic aids helps a lot, but closing the gap between tractor and trailer still offers the biggest savings. He also pointed out that "thicker" air density (cold temperatures) adds to resistance and is a big factor in cold climates.
"January and February are statistically the worst; June and July the best," he said.
Detroit Diesel's Chuck Blake said a 10-degree change in temperature can impact fuel economy by 2%. He said cold-weather fuel blends - with a high percentage of No. 1 diesel and/or kerosene - drops fuel Btu's up to 3%. The lower the Btu's, the more fuel it takes to produce the same engine horsepower. Refineries and fuel distributors blend diesel according to anticipated cold temperatures. This prevents or reduces fuel waxing that can cause filter plugging. It's also why many cold-climate fleets install fuel heaters.
Several years ago, a large common carrier fueled some of its trucks in Memphis and ran to St. Paul where it was sub-zero when they arrived. The Memphis fuel was a straight No. 2 and the fleet's trucks wouldn't start the next morning in Minnesota. Fuel lines were plugged solid.
Blake concluded that combining all the cold-weather factors discussed - in a worse-case situation - fleets could see as much as a 17% decrease in miles per gallon when Old Man Winter hits hard. He broke it down this way: Fuel, up to 3%; lubricants, up to 3%; tires, alignments, driver idling etc., between 10% and 15%.
So, as winter arrives for many of us, beware of the mpg consequences and work on those issues that will help your fleet the most. Accept the fact that cold temperatures breed higher fuel bills.
