Fiat Chrysler introduced the fuel-sipping Ram EcoDiesel HFE in Detroit on Tuesday. Source: Fiat Chrysler Automobiles.

Ford's (NYSE:F) new aluminum F-150 might be the hot new pickup on the block, but Fiat Chrysler Automobiles (NYSE:FCAU) isn't backing down.

FCA's truck mavens dropped a surprise at the North American International Auto Show in Detroit on Tuesday: a new, even-more-efficient version of their Ram 1500 EcoDiesel, the only light-duty full-size pickup offered in the U.S. (at the moment) with a diesel engine.

The new Ram EcoDiesel HFE (for "High Fuel Efficiency") sports an EPA highway rating of 29 miles per gallon. What does that do to Ford's EcoBoosted strategy to win truck mileage supremacy?

Ford doesn't want to talk about the F-150's fuel economy right now
For right now, at least, Ford is responding by changing the subject.

Ford North America chief Joe Hinrichs and other company officials in Detroit this past week emphasized that the new F-150's lighter weight offers benefits to truck buyers that go well beyond fuel economy. All other things being equal, they pointed out, a lighter truck can haul more and tow more -- two attributes of immense importance to many truck buyers.

Ford is aggressively promoting the all-new F-150 in a series of TV ads. The ads tout the truck's reduced weight and increased towing capacity, but don't mention fuel economy. Source: Ford.

That's all true. And it's a way to deflect mounting criticism of Ford's mileage strategy, as some road tests of the new F-150 and its 2.7-liter EcoBoost V6 have failed to show the fuel economy the automaker has claimed.

When Ford first released the new truck's fuel-economy numbers, many were surprised to see the company's massive, expensive "lightweighting" effort had not yielded better ratings. Specifically, we were surprised to see Ford had not beaten the diesel Ram.

And now Fiat Chrysler has made the margin a bit wider.

The new Ram has big highway mileage numbers -- and a big price tag
The new Ram 1500 EcoDiesel HFE gets an EPA-rated 21 miles per gallon in the city, 29 highway, and 24 combined. That's a slight increase over the regular EcoDiesel, thanks to some aerodynamic optimization. It compares with Ford's best of 19 city, 26 highway, and 22 combined for a 4x2 version of the new F-150 equipped with that 2.7-liter EcoBoost V6.

It also comes with brawny towing capacity and diesel bragging rights. But you'll pay for the privilege: The EcoDiesel is a $4,000 option on the Ram 1500.

Ford argues that the high price of the EcoDiesel option -- plus the price of diesel fuel versus gasoline -- still makes the gas-powered Ford the better bet for cost-conscious buyers.

Gasoline and diesel prices have both fallen over the last few months, but gas has fallen much farther. According to U.S. government statistics, the average price of a gallon of diesel fuel as of this past Monday was $3.053, versus $2.139 for a gallon of regular unleaded gasoline.

For an average buyer driving 15,000 miles a year, that still makes the F-150 cheaper to operate, Ford says. And for now, anyway, the numbers bear the company out. 

The upshot: With $2 gas, Ford is still holding a good hand
That cost advantage should leave Ford in a solid position with cost-sensitive commercial truck buyers, despite the Ram's advertising-friendly EPA rating advantage. While we'll have to wait until Ford's dealers have full inventories of the all-new F-150 to know for sure how it will sell, early signs continue to bode well for the Blue Oval's new pickup.