Ford is in serious trouble. It’s selling more pickup trucks than it has at any time in its 100-plus-year history. In fact the entire auto industry has seen a huge surge in truck sales.

Just look at the market share of “light trucks” over just the last 35 years. It was just 9.7 percent in 1979; then it started a steady climb to 47 percent by 2001. Since then it’s remained north of 50 percent each year, with no signs of slowing down.

So why is this a problem for Ford? It all has to do with CAFE—and not the one you lunch at. We’re talking “Corporate Average Fuel Economy.”

The auto industry’s CAFE standards require each vehicle hit a government-mandated fuel economy rating—small cars have to do more, trucks less. However, averages are calculated across the sales of a company’s entire fleet, and if you sell a lot of trucks, well, your averages are going to go up, aren’t they?

CAFE rules were first implemented by the government after the Arab oil embargo of 1973 and ’74. U.S. energy policy demanded automobile manufacturers do their bit to make the USA energy self-sufficient.

Today that goal remains the same. Currently CAFE covers a manufacturer’s fleet of passenger cars or light trucks produced for sale in the United States with a GVWR (gross vehicle weight rating) of 8,500 lbs or less (though medium-duty passenger vehicles, such as large sport-utility vehicles and passenger vans, with GVWRs of up to 10,000 lbs are also included). We follow the same rules in Canada.

So even though once-gas-thirsty pickup trucks have kept up with technology, the volume of sales has increased dramatically, upsetting the fuel economy balance at companies like Ford.

Powertrain upgrades in past decades were mostly driven by market forces, but today looming new CAFE rules are dictating what companies like Ford will have to build.

At the 2017 North American International Auto Show in Detroit, Ford gave us a clear glimpse of this CAFE-fueled future. The company announced it’d add a small diesel to its F-series pickup this coming year, and by 2020 would offer a hybrid-powered version of the truck.

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Ford is currently also pairing its EcoBoost engines to 10-speed automatic transmissions and reducing their displacement, despite the fact these motors are already very cheap on fuel.

It’s obvious all these efforts are aimed at decreasing fuel consumption, which in and of itself is good for consumers. It’s the speed at which automakers have to make these efforts that’s worrisome.

But they have no choice—CAFE is the law, and the wild card for Ford is that the ever-increasing ratio of truck-to-car sales can’t be predicted. They have to find a way to bring the fleet average down.

Targets for CAFE 2016 fuel economy were a pretty steep 34.0 mpg (6.9 L/100 km) across the fleet, which why Ford – and other truck builders – are stretching fuel economy by whatever means possible. Adding diesel powertrains are a great way to further bring that average down, but gas-electric hybrids would lower it even more.

We’re here in the early days of 2017, but Ford must look much farther ahead because the CAFE goals keep changing. For instance, an F-150 or equivalent-sized pickup truck has a 2018 target of 25 mpg (9.4 L/100 km). But in 2022, that number’s going up to 26 mpg (9.0 L/100 km) and then rises again each year until it hits 30 mpg (7.8 L/100 km) in 2025.

It’s that day, not as far into the future as you might think, that Ford engineers already have marked in their calendars.