For decades, fuel economy has been one of the most important numbers in trucking. But as fleets prepare for model year 2027 equipment, miles per gallon may no longer tell the entire story.
New EPA emissions requirements beginning with model year 2027 establish significantly more stringent nitrogen oxide (NOx) standards for heavy-duty engines. As manufacturers introduce new engines and after-treatment systems to meet those requirements, fleets should be asking an important question: Should we still be looking at fuel economy, or should we be looking at total fluid economy?
Fuel Economy vs. Fluid Economy
Traditionally, fuel economy centers on diesel. But modern diesel trucks also consume diesel exhaust fluid (DEF), which is used by selective catalytic reduction (SCR) systems to control NOx emissions.
With EPA27, manufacturers are developing different engine and aftertreatment strategies to meet new requirements. Those strategies could affect both diesel and DEF consumption.
That’s where fluid economy becomes important.
Instead of evaluating diesel consumption alone, fleets should consider the combined consumption and cost of diesel + DEF to understand what it actually costs to operate a truck.
Cummins, for example, has publicly discussed “fluid economy” when describing its 2027 engines and aftertreatment systems. The company says its 2027 X15 is expected to maintain DEF consumption similar to its 2024 X15, depending on duty cycle.
The takeaway isn’t that EPA27 will automatically increase DEF consumption. It’s that not every truck will achieve compliance in exactly the same way, and fleets need to understand what those differences mean for operating costs.
Small Differences Can Mean Millions
Fuel remains one of the largest operating expenses in trucking. According to the American Transportation Research Institute’s 2026 operational-cost data, fuel accounted for approximately $0.482 per mile (20.6%) in 2025.
That means even a small difference in fluid economy can become significant at scale.
Consider a difference of just one cent per mile:
100,000 miles = $1,000 per truck
500 trucks = $500,000
1,000 trucks = $1 million
And that’s after only 100,000 miles per truck.
Extend that difference across the fleet’s first ownership cycle, and seemingly minor differences in diesel and DEF consumption can potentially become millions of dollars.
Don’t Forget About Idle Time
EPA27 could also give fleets a reason to revisit the economics of idling.
The North American Council for Freight Efficiency estimates that a typical Class 8 truck can consume around 0.8 gallons of diesel per hour while idling, while a modern diesel APU consumes roughly 0.25 gallons per hour.
If diesel and DEF consumption during idle operation changes with new equipment, the return on investment for idle-reduction technologies such as APUs and eAPUs could change as well.
Fleets may also want to reconsider programmable engine parameters, automatic shutdown settings, and other idle-reduction strategies.
The point isn’t that EPA27 will automatically make these technologies more or less valuable.
It’s that fleets shouldn’t assume the old calculation still applies to the new equipment.
Measure Before You Buy
A fleet purchasing hundreds or thousands of tractors isn’t simply buying trucks. It’s committing to millions of miles of future diesel consumption, DEF consumption, maintenance, and operating costs.
The truck that looks best on paper isn’t necessarily the truck that will deliver the lowest cost in real-world operation.
That’s why fleets preparing for EPA27 should be asking more than: “What MPG does this truck get?”
They should also be asking: “What does it cost us in total fluids to operate per day, in miles and idle?”
If investing a few thousand dollars in testing and analysis before a major equipment purchase can help identify hundreds of thousands, or potentially millions, of dollars in operating-cost differences, it’s worth asking those questions before the purchase order is signed.
Mesilla Valley Transportation Solutions helps commercial fleets make confident operational decisions through scientifically validated fuel economy testing, fleet consulting, and implementation strategies developed through decades of real-world fleet experience.
Preparing for EPA27 equipment purchases? Contact MVTS to understand what diesel consumption, DEF consumption, idle performance, and total fluid economy could mean for your fleet before you make the investment.