Why the government won’t tell you which automaker had the most efficient 2013 models

In an industry that fights for every last sale, claiming the title for being the most fuel-efficient automaker in the country has traditionally been a huge win. Even in times like these when gas prices have eased, shoppers often compare mileages as much as horsepower, and when gas prices soar — as they inevitably will — having that message at the ready gets more tire-kickers into dealerships.

In the United States, it's a federal report that typically determines which automaker may call itself the most fuel-efficient. Yet this year, the U.S. Environmental Protection Agency went out of its way to deny that accolade to the automaker with the highest average efficiency from vehicles it sold in the 2013 model year: Hyundai.

The South Korean automaker has itself partly to blame: After touting its fuel efficiency for years, the company admitted in November 2012 that it had incorrectly measured the fuel economy of 900,000 Hyundais and Kias, forcing it to write down the MPG values on 13 models. That sparked an ongoing EPA investigation into how automakers report mileages, since the EPA often checks only a fraction of the figures they estimate; the flaw that forced Ford to perform a similar reduction on the C-Max Hybrid.

Hyundai and Kia's corrected data has been accepted by the EPA and worked into its annual report on fuel economy, including its proclamation that the fuel efficiency of new U.S. cars and trucks has never been higher, averaging an estimated 24 mpg for 2013 models. But when the EPA released its breakdown of average efficiency by automakers, it didn't include Hyundai and Kia in the published table, citing the open investigation — choosing to give top marks to Mazda and Honda instead.

Here's the real list of 2013 fuel efficiency, drawn from the EPA's own data and automaker's sales estimates. This compares top-level makes that sold at least 40,000 vehicles, so for example Volkswagen includes Audi:

While these numbers are records, the industry still has much further to go to meet increasing standards, which in the United States would require an overall average near 39 mpg by 2025. It's the toughest engineering challenge facing automakers today, driving hybrids into supercars like the LaFerrari and the Porsche 918, and the revival of three-cylinder engines in cars like the Ford Fiesta and the Mitsubishi Mirage.

Earlier this week, General Motors announced it would retool a factory to build a 10-speed automatic transmission, leapfrogging Chrysler's 9-speed just introduced on the Jeep Cherokee. The more gears a transmission has, the more it can keep an engine running at peak efficiency — and stuffing gears into a transmission offers a lower-cost way to boost mileage than by adding an electric motor and battery.

But it's not quite fair to compare a small-car company like Mazda with a Detroit giant that sells hundreds of thousands of pickups every year; a small car has far less metal, plastic and heated/massaging leather to haul around than a luxury SUV. Thankfully, the EPA has been collecting a few more pieces of data that give a fuller picture of how well individual automakers are coping:

Take Mazda as an example: It's relied on squeezing the most it can from gasoline engines and cutting weight to increase efficiency, giving it the lightest yet slowest vehicles among new 2013 models. Hyundai and many others have shrunk their engines while maintaining or boosting power through turbocharging or other technology; luxury makes like BMW and Mercedes-Benz parent Daimler suffer because of their relatively hefty models and high-power engines.

Yet it's that last column that's most revealing. It's the EPA's calculation of test MPGs times average weight in tons — an attempt to measure how efficient an automaker's vehicles are controlling for weight. This helps the Detroit makes; although Chrysler lags, GM improves to mid-pack and Ford stands as the second-most fuel-saving carbuilder, thanks to its Ecoboost engines. But Toyota runs away in this measure, mainly due to 30 percent of its 2013 sales coming from hybrid models, from the Prius to the Highlander. Expect more automakers to follow this route as 2025 draws nearer.