If you're above a certain age, you have vivid memories of the 1973 oil embargo and the resulting fuel shortages. Many people were inconvenienced, frustrated, and angered by empty gas pumps and long lines.

And perhaps for the first time, many people thought seriously about fuel consumption as both a budgetary and a freedom of movement concern. Lacking safe, efficient, convenient, flexible public transportation outside of major cities, we were hopelessly dependent on private cars and trucks. Without gas, we couldn't get where we wanted and needed to go. The gas crisis temporarily caused vehicle preferences to change. Small, fuel-efficient cars sold out, and almost no one wanted big cars and trucks. Then fuel became plentiful again, gas prices dropped, and some people who had downsized to smaller vehicles wished they hadn't.

That cycle repeated itself in 1979-1980 and again in 2008-2009, but the lasting legacy of that 1973 fuel shortage is the federal government's response: Corporate Average Fuel Economy laws. The first required automakers' 1978-model "sales-weighted fleet averages" to be no lower than 18 mpg -- no challenge for imports, which produced a lot of small cars, but a tall order for domestics. It meant makers had to balance sales of profitable larger vehicles with sales of smaller ones (usually at a loss), whether or not anyone wanted to buy them. Light-truck standards followed for 1979, beginning at 17.2 mpg for 2WD models and 15.8 for those with 4WD.

When it comes to CAFE, critics contend it's a sorry substitute for reducing fuel usage through higher fuel taxes, as other countries have done, because it puts the onus on automakers, regardless of market demand, and drives up vehicle prices. Nevertheless, these laws were toughened each year through the early 1980s, softened slightly in the mid-'80s, then leveled at 27.5 mpg for cars from 1990 to 2010. The truck number accelerated slowly to 20.7 mpg (combined for 2WD and 4WD) for 1996, stayed there through 2004, then climbed again to 23.5 mpg for 2010.

Then, on May 19, 2009, President Obama announced a new "national fuel economy program" mandating a fleet average of 35.5 mpg for by 2016, a daunting 29-percent increase that moved the requirements of an existing 2007 law forward by four full years. Since 2012 models were essentially done, automakers would have just four model years to achieve it.

A year later, he ordered the U.S. National Highway Traffic Safety Administration, which (for some reason) manages CAFE, and the Environmental Protection Agency to jointly mandate much tougher standards for 2017-2025. They laid out four scenarios of 3-, 4-, 5-, and 6-percent annual increases over that 35.5-mpg 2016 level. The 6-percent-per-year schedule would have resulted in a CAFE of 62 mpg by 2025, but they later settled on a somewhat more reasonable 54.5 mpg.

Using prices for an array of fuel-efficiency technologies projected by a 2010 National Research Council study commissioned by NHTSA, the Center for Automotive Research predicted a retail cost of meeting a (then-proposed) 56-mpg CAFE at $6714, which (when added to an estimated $1500 increase for future safety requirements) would result in 2025-model vehicle stickers $8214 higher on average than 2016 prices.

The only good news for automakers in these new requirements (which were not fully formulated for 2017-2025 as this was written) is that they move from a straight fleet average to a much more complex but more rational footprint-based formula, which comprehends the basic law of physics that larger vehicles will consume more energy than smaller ones. (A vehicle's footprint is its wheelbase times its average tread width.)

It's important to note that CAFE-compliance fuel-economy numbers, derived from EPA emissions-tests, are higher than those you see in ads and articles and on window stickers because the feds mathematically adjust the test results downward to obtain the published numbers that are better predictors of real-world efficiency. Thus a 2012 subcompact Honda Fit with a 40-square-foot footprint must achieve an EPA test fuel economy of 36 mpg, equivalent to a published 27 mpg, while a Ford F-150 with a 65-75-square-foot footprint must test at 22 mpg for a published 17 mpg.

We asked experts from Ford, Chrysler, and Toyota (GM declined an interview) what this means to full-size truckmakers.

"There is no fixed number that we will have to hit," says Jeff Lewis, head of Ford's Vehicle Energy Management Engineering group. "[Compliance] will be tied to the footprints and the overall mix of our products." He added that the light-truck target will be significantly lower than the 54.5 mpg required of the entire U.S. fleet.

"The input we have been providing [internally] is, 'Here is what our cycle plan looks like, here are the footprints of those vehicles, and here is the predicted sales mix of those vehicles.' Our CAFE planning team comes back and says, 'Then here is what each of our vehicles needs to deliver for us to be compliant.' The footprints and mix will determine what our corporate number needs to be, then we will know where each of our products needs to be."

"A given vehicle is assigned a target," explains Gary Oshnock, Chrysler's manager of Fuel Economy-CHG Programs and Regulatory Affairs. "What a manufacturer does to reach compliance is sales weight. Each model will have X, Y, or Z number of sales to come up with an average footprint for its fleet. And for a given model year, that average footprint determines the fleet average fuel economy requirement.

"We use an SAE procedure to determine the wheelbases and track widths," Oshnock says. "But when we get into heavy-duty trucks, it will be a completely different system. A regulation just came out for 2014-2018 model years for over-8500-pound-GVW trucks -- wreckers, ambulances, potato-chip trucks, up to Class 8 line-haul trucks.

"Instead of a footprint, Class 2B and 3 full-size pickups and vans will use a 'work factor' attribute: 0.75 times payload capacity (GVW minus curb weight), plus 500 pounds if it's equipped with 4WD, plus 0.25 times tow capacity (GCW minus GVW). But Chrysler supports this methodology because it recognizes the underlying physics of a truck's capability."

Michael Cairns, Chrysler's vehicle line executive for Ram Trucks and Regulatory Affairs, points out that truck makers will have to meet four separate requirements, two for light-duty and two for heavy-duty, because EPA's "greenhouse gas" mandates (as of now) are different from NHTSA's CAFE requirements. On top of that is another set of CO2 requirements for California and 13 other states that have adopted California's standards.

"The government has tried, with industry help, to tie those together," Cairns says, "to provide some rationality among them." He points out that light-truck fuel-economy standards apply to a lot more vehicles than just pickups, including minivans, Jeep Wranglers, and even Patriots, since NHTSA and EPA categorize trucks based on attributes such as cargo-carrying and off-road capabilities. Oshnock continues that, while two government agencies and California are involved, "the programs are fairly well aligned -- though it does add complexity. Trying to get to a single national program? We generally support this effort."