Drilling down on MPG

Each group had between eight and 12 companies included and an average fleet size of 250 trucks with either 13L or 15L engines, according to Vusion, and 95% of the trucks included were company-owned. The data set presents "physical mpg" calculated from fuel purchases and odometer readings for each particular truck.

Also, outlier data — measures at the extreme upper or lower ends of the group — were excluded, "because we don't want to have any weird data," said Anne Heitkamp, a Vusion statistician. "So we cleaned that up."

The benchmarking data span a three-year period from the second quarter of 2012 to the second quarter of 2015. At the start, the average fuel economy of all the fleets studied was 6.44 mpg, which climbed to 6.99 mpg at the second quarter of 2015.

"MPGs are on the rise. From quarter two of 2012 to quarter two of 2015, MPGs are up 8.5%," said Vusion President Thomas Fansler. "That's a remarkable increase." However, that's only the broader, aggregate picture, he noted, discussing a number of factors fleets should be aware of that affect what their expected fuel economy should be.

Emissions tech and real-world mpg

For starters, an engine's emissions technology is a differentiator, according to Fansler. Engines built from 2007 to 2009 used EGR, he noted, and SCR technology was added in 2010.

"SCR paired with EGR gives the OEM engineers more flexibility in how they optimize for torque and horsepower. It results in more efficient engines — higher cost as well, but also more efficiency," he said.

Breaking things out by engine type, Vusion's data show that newer, more advanced SCR/EGR engines in the benchmarking group were getting an average of 6.69 mpg in quarter two of 2012 and 7.07 mpg in quarter two of 2015, or a 5.7% increase. Older, pre-2010 EGR engines, meanwhile, went from an average of 6.05 mpg at the start of the study period to 5.97 mpg in the second quarter of 2015, a decline of 1.3%.

"Those EGR vehicles are going down in fuel economy as they age," Fansler told the audience. He said there was about a 0.5 mpg difference in physical fuel economy between EGR trucks and SCR/EGR trucks as of quarter two of 2012, "and the spread is even wider as those EGR trucks have become older. Now we're talking about SCR trucks that are operating up around 7 [mpg] and EGR trucks that are operating below 6."

The two trends driving the overall rise in fleet mpg are the improvement afforded by SCR technology and the retiring of older EGR-only trucks, Fansler explained: "It's the combination of those two things that's resulting in the biggest change."  

There's also a difference in electronic control module (ECM) data from EGR and SCR/EGR trucks, according to Vusion. ECM data on fuel economy are based on algorithms and provide an estimate of mpg, Fansler contended, "and sometimes those algorithms don't track true to the physical world."

"It's important when you're trying to measure your fleet to not get too set on using ECM MPGs without understanding the level of accuracy of that data you've got," he said. EGR-only trucks, for example, "had a horrible tendency to overstate fuel economy" by understating gallons of fuel burned, according to Fansler, which tended to produce a 10-13% overstated fuel economy from the trucks' ECMs.

To a lesser degree, newer SCR/EGR trucks also overstate fuel economy and understate gallons of fuel consumed, Fansler pointed out, but they're getting closer to reporting actual physical mpg. "With the newer SCR trucks, we're seeing those lines finally converge," he said. As of quarter two of 2015, he contended that SCR trucks are producing an average of about a 2% overstatement in ECM data vs. physical fuel economy.

"So if you're using ECM data for mpg, you might want to consider a small adjustment. It does vary a bit based on manufacturer," Fansler noted. "But the good news is the trend is overwhelmingly toward much more accurate reports coming off those engines."

Other factors in the equation

Fansler and Heitkamp discussed other factors fleets should take into consideration when determining where to benchmark their fuel economy. "Each of you drives different vehicles, and we know they get different mpg," Heitkamp said.

In its benchmarking group, Vusion found truckload dry van carriers got the best fuel economy, followed by refrigerated transporters, LTL carriers, flatbed trucks and finally tanker trucks. Fansler noted that the ordering reflects only the experience of its customers and is not necessarily the case across the industry; still, in quarter two of 2015, Vusion's truckload dry van carrier group averaged 6.99 mpg, while the tanker truck group averaged 6.03 mpg.

Another big factor in fleet fuel economy is season, according to Vusion. "Quarters one and four are affected by colder temperatures. That affects your mpg because it has more drag on the truck, lowering your mpg," Heitkamp explained. "The second factor in seasonality is idle," she added. "In quarters two and three — the hotter temperatures — those trucks need to idle, which also affects your mpg" because it burns fuel but doesn't add any miles traveled.

Aggregating data for the 2012-2014 period, Vusion found an average seasonal effect on fuel economy of negative 2.8% in quarter one, a 2.6% increase in quarter two, a 0.7% increase in quarter three and negative 0.5% in quarter four. "This is just our sample," Heitkamp said. "You might see some different numbers, but this is just a benchmark to see what you should adjust for."

Within seasonality, Vusion also observed a regional modifier in the data. For the benchmarking study, the company considered a fleet northern if it runs 80% of its miles in the northern, upper tier of the United States, or southern if 60% or more of its miles are in the southern, lower tier of the United States.

For southern fleets, the seasonal effect on mpg for quarters one and four "flattens out somewhat," Heitkamp said, with less of a negative effect on fuel economy due to milder temperatures in winter. But southern fleets need to idle more during quarters two and three, bringing down the mpg increase that would come then. It's the reverse for northern fleets: they see more of an mpg drop in quarters one and four due to colder temperatures, and more of an mpg increase in quarters two and three with less need for idling.

Fansler also pointed out differences by engine displacement and explained how fleets should adjust their mpg expectations. He said the benchmarking groups had "about an equal split" between 13L and 15L engines, "so when we're looking at these benchmarks, you can think of it as sort of a 14L engine," he said. However, Vusion provided a fuel economy benchmarking adjustment for 13L or 15L engines.

"What we found was the 13L fleets could expect to run an average of 0.03 mpg higher than the [aggregate mean mpg], and the 15L engines could expect to run an average of 0.03 lower," Fansler told the audience. "There's going to be a little more variance based on certain factors like manufacturer and how you spec your vehicles, but in aggregate there's about a 0.06 mpg spread."

Other operational factors to weigh include the types of roads a fleet's trucks drive on, he noted. "When we're talking about over-the-road fleets, many of them have a small amount of local pickup and delivery operation. Trucks operating at lower average speeds perform differently," Fansler said.  The adjustment boils down to fleets operating on local or regional roads vs. fleets that operate primarily on highways.

"If you've got a lot of primarily local and regional trucks in your fleet, you would have an expectation that your mpg would perform about 5% worse than the benchmark levels," contended Fansler.  

Load weight is part of it as well, according to Vusion. Fansler said a general rule of thumb is that every 5,000 lb. more than 30,000 a truck is hauling will reduce expected mpg by about 1.5%. Lighter loads, by contrast, increase expected fuel economy.

On that note, Fansler explained that the benchmarking groups had an empty-loaded ratio of about 12%. Fleets that have higher percentages of empty miles can expect to see higher overall fuel economy, he said, while those with lower percentages can expect lower.

Visit www.vusion.net for more information.