Ford says it has received official EPA fuel economy numbers for the new 3.0-liter Power Stroke diesel, and the figures are good if you get the 4x2. But if you want power to all four wheels and you’re not a fleet customer, the numbers are much, much lower.

Hop over to Ford’s , and you’ll read that “The 2018 FordF-150’s first 3.0-liter Power Stroke diesel engine officially boasts EPA-estimated ratings of 30 mpg highway, 22 mpg city and 25 mpg combined,” and that that those EPA numbers are the highest of any full-size truck.

But a Ford representative told me over the phone that those MPG figures—which won’t be on the EPA’s official website until May—only apply to 4x2 models, and the that the four-wheel drive version of this same truck scores an EPA rating of 25 MPG highway, 22 MPG combined and 20 MPG city.

That’s five MPG lower on the highway just for checking the “4x4" box.

Looking at other diesel vehicles like the , , and for reference—and also looking at —the difference in highway fuel economy between 4x2 and 4x4 versions of the same truck is usually around one and two MPG. I can’t say I’ve ever seen a gap as high as five.

I reached out to Ford to learn more, and a company representative told me that it comes down to how the vehicles are configured. The 4x2 is a SuperCrew with a tall 3.31 fuel economy-oriented final drive ratio and all-season tires on 20-inch wheels. The 4x4 is also a SuperCrew, but it gets all-terrain tires on 20-inch wheels, a towing-oriented 3.55 final drive ratio, and a Torque on Demand transfer case.

The 4x4's all-terrains likely add rolling resistance, and the shorter final drive ratio likely brings the engine to a less efficient operating point during highway cruising.

The four-wheel drive system, with its added weight, increased drivetrain losses and perhaps more protruding hardware (which could affect aerodynamics) surely also contributes to the lower fuel economy. The Torque on Demand system is automatic, which I suppose could sap a bit more power than a driver-operated system (even though modern full-time four-wheel drive systems are fairly efficient).

It’s worth mentioning that Ford says its fleet-only XL and XLT 4x4 models—which come with shift-on-the-fly transfer cases—score a decent 28 MPG highway, 24 combined, 21 city. Those trucks, like the retail 4x4s trims, come with all-terrain tires, though unlike the retail trucks they have the taller 3.31 axle ratio.

But if you’re not a fleet customer, the highest fuel economy you’ll get in the 4x4 Ford F-150 3.0-liter Power Stroke diesel is 25 MPG, and if you’re okay with only two-wheel drive, you can buy a truck rated as high as 30 MPG highway.

I must say I’m surprised that, according to Ford, this delta comes down to just tires, final drive ratio, drivetrain efficiency and weight of that drivetrain. Other automakers offer trucks with the same motors but with different final drive ratios, different tires and different drivetrain options, and yet I’ve never seen such a huge delta in EPA estimated highway fuel economy between variants of the same truck, especially without significant differences in aerodynamic treatments.

A Ford representative told me he’s going to send me a statement on the topic, so hopefully we’ll learn more soon.