Accurately evaluating heavy-duty freight efficiency requires looking beyond simple MPG to complex metrics like freight-ton miles per gallon and fluid economy, which account for payload, multi-fluid consumption, and varying fuel types.
- MPG Ignores Work Performed: A loaded class-8 semi-truck hauling 46,000 lbs may yield the same 10 MPG as a vintage sports car carrying a toothbrush, demonstrating why cargo weight (freight-ton efficiency) is critical to understanding actual productivity.
- Measurement Accuracy Degrades in Practice: Standard distance and volume calculations carry built-in inaccuracies due to tire tread wear altering odometer calibration, telematics data compression, and fuel volume changes caused by diesel temperature fluctuations.
- Modern Powertrains Require Complex Fluid Tracking: Evaluating diesel efficiency now requires measuring diesel exhaust fluid (DEF) consumption alongside fuel—termed "fluid economy"—while alternative fuels like CNG, hydrogen, and electricity require new conversion metrics like DMPGe.
- Simplicity Continues to Trump Precision: Despite decades of regulatory work (EPA SmartWay, GHG Phases I & II) developing rigorous standards, fleets and drivers still rely heavily on MPG because it is intuitive, accessible, and easy to communicate.
I always have loved measuring reservoirs in units of acre-feet of water. Whoever invented that priceless measurement should get an award. Most of us measure volume in terms of liters or gallons. But acre-feet has so much more character.
Your diesel truck measures performance in terms of miles per gallon. That is so straightforward. Even if your truck doesn’t know that, you know how many miles you drove and how many gallons of fuel you bought, so its pretty easy to calculate MPG.
MPG is a great metric. It defines vehicle efficiency. Only it doesn’t.
A Volvo heavy-duty truck may get 10 MPG, the same as a 1990 Lamborghini Countach. Okay, one is diesel and the other is gasoline, you got me there. Both have room for two people, so they are equivalent in moving two people down the road. Only one is carrying maybe 46,000 lbs. of paid freight, while the other might be carrying a toothbrush.
Back in the early 2000s when the OEMs, fleets and the EPA were wrestling with how to emphasize better fuel economy as part of starting the SmartWay program, the challenge was agreeing on useful metrics to measure performance. They realized that MPG probably wasn’t quite enough. The amount of freight carried seemed to be absent. The term freight-ton efficiency, or freight ton miles per gallon came into vogue.
Simply put, the freight ton miles per gallon is simply the amount of freight carried times the MPG. The greatest challenge is that freight weight is not all that precisely known, and it constantly varies from trip to trip. A dedicated freight truck may go out full with 46,000 lbs. of freight and a gross vehicle weight (GVW) of 80,000 lbs. but the return trip may be empty, zero freight tons, and a GVW of 33,000 lbs.
Every metric always has qualifiers. They get pretty involved when caring about the absolute accuracy and precision of the measurement. Even accuracy and precision often get confused — accuracy is how close the measurement represents reality and precision is how well the measurement can be repeated.
Freight efficiency, the shortened version of the term freight ton miles per gallon, is complicated. Complicated measurements always cause problems. Does the measurement apply per load, per day, per year? How does it account for different loads, different drivers, different routes?
The complexity gets worse when we look at the MPG part of it. What gallon is being measured? Sure, diesel, but diesel at what temperature? Turns out gallons are a measure of volume, and diesel volume changes based on temperature. The people caring about accuracy and precision go to a great deal of trouble to accurately measure the temperature of diesel before they state MPG numbers.
Miles is another issue. Turns out your odometer has to guess at your tire diameter. Your tires constantly wear, so a new tire is larger than a worn one. The circumference of that tire is what determines your odometer clicking over the next tenth of a mile. Some engineer picks a representative value for the recommended tire size for your truck, and that’s what the odometer uses forever. It’s simplifying things, but in the process, it’s less accurate, although probably relatively precise.
What about telematics systems? Don’t they use global positioning system (GPS) satellite data to measure distances accurate to inches? Well, yes and no. Turns out data has costs, so often telematics systems using GPS may average data over periods of travel. Or the telematics system may just be defaulting to the on-board odometer on the truck. The answers are usually good enough.
The other part of gallons that is confusing in modern trucks is that diesel trucks use a combination of diesel fuel and diesel exhaust flued (DEF). Every truck may use different amounts of DEF with different amounts of diesel. This is called dosing and varies considerably based on many factors.
The people that care greatly about both accuracy and precision came up with the term fluid economy to reflect that the diesel engine is using two fluids and we should care about the amounts of both when looking at net vehicle efficiency.
Now diesel engines can run on natural gas. Another complication since natural gas is measured in terms of pressure, pounds per square inch (PSI) or atmospheres (ATM) or other similar terms. The gauges in a CNG powered truck likely are not measuring in MPG. Empty, for a CNG powered truck, is when the pressure in the tanks drops below a specific point. That pressure depends a great deal on temperature. Gases expand a lot with higher temperatures, or reduce with lower ones.
And fuels for trucks are available now as electrons, or hydrogen molecules, sometimes even propane and even gasoline is making a resurgence. The test people invented DMPGe — diesel miles per gallon equivalent — as a measure and as a common unit to compare all these different fuels.
The natural conclusion from all this is that the critical performance efficiency metric should be freight ton diesel and DEF fluid miles per gallon equivalent. Whew. That’s a mouthful.
In the 25+ years since I first was involved in discussions on measuring truck efficiency, fleets and drivers seem to just default to MPG as the measurement they talk about. It’s an easy number to grasp, to measure and to state. It is not accurate, nor particularly precise when comparing vehicles, drivers, routes, loads, etc., but it’s easy.
For a detailed discussion of fluid economy, you can see the SAE J1321 Type II fuel efficiency standard. The freight ton efficiency measurement is described in a variety of government regulations including the GHG Phase I rules from 2011 and Phase II from 2016 rules and details for the SuperTruck 1, 2 and 3 programs (a summary of that is in NACFE’s free report SuperTruck 2: Empowering Future Trucking).
For some background on the complexity of measuring fuel efficiency, see NACFE’s free report Determining Efficiency. I wrote this report in 2014 when I first joined NACFE, trying to capture my corporate and standards experiences in this field, and it’s still largely relevant today.
What metric do you use? I think I will start using freight ton miles per acre foot of equivalent diesel and DEF, just to be different.
