Why Manufacturers Keep Correcting Their Fuel-Economy Estimates

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Car & Driver  /  September 29, 2016

The EPA uses dyno cells like in the image above for tests from 20-degree cold starts to 86-degree highway runs. A separate cell with high-intensity lights simulates the solar load of a sunny day for the high-temp tests.

Recent cases of “misstated” fuel economy involving Ford, GM, Hyundai, and Mercedes-Benz have this in common: All were caught by the EPA during audits of data submitted by the manufacturers for certification. That’s because the EPA—lacking the personnel, facilities, and funding to test every vehicle—lets the manufacturers largely certify themselves and just submit the data.

All automakers follow the same set of procedures designed to generate comparable mpg numbers, but manufacturers have obvious incentive to exploit gray areas and loopholes to produce higher ratings. The goal is to net the best window-sticker numbers without netting ones so good that they raise suspicions. It’s a delicate dance, and engineers we spoke with described it as pervasive. These are a few manufacturer approaches:

It’s a Drag

The EPA-prescribed dynamometer tests incorporate calibration settings intended to align laboratory results with real-world performance. Road-load coefficients—determined from a coast-down test wherein a car coasts in neutral from 80 mph—are programmed into the dyno to account for the energy lost to aerodynamic drag and friction. Recently, the EPA had to remind automakers to test cars on typical road surfaces and with critical components such as brake pads in place—their removal lowers friction. The EPA also warned against “adjustments,” such as special alignments or using extra-worn parts. Mi­tsu­bishi is currently being investigated for manipulating coast-down data, and Hyundai had to lower the ratings of 1.2 million cars in 2014 after it cherry-picked favorable figures instead of using averaged data.

Body Swap

In 2013, Ford used a legal EPA provision allowing cars of similar weight and with the same powertrain to share ratings, assigning the C-Max hybrid the same 47-mpg combined figure as the Fusion hybrid. Customer backlash and an EPA audit revealed real-world C-Max fuel economy to be far lower, however, prompting Ford’s “voluntary” reduction of the hybrid wagon’s rating. For a time, Toyota used the same clause, without issue, to apply the Camry’s fuel-economy rating to the larger Avalon.

Popularity Contest

Per EPA rules, only the expected sales leader within a model family sharing a powertrain must be tested. This explains why, out of the four available trim levels on the 2016 Chevrolet Cruze—two with multiple transmission choices—only the automatic LS has a smaller, 12.1-gallon fuel tank. A smaller tank means less weight. GM used this lighter LS model to generate the 30-mpg city/42-mpg highway ratings for multiple automatic Cruzes, a group that includes the better-equipped LT that rides on wider 16-inch wheels, fills the LS’s empty trunk well with a spare tire, and sits in a higher EPA test-weight class. Legally, Chevrolet could apply the superleggera LS’s ratings to the heavier, range-topping Premier, too, but it tested that one separately and rated it at a lower 30 mpg city/40 highway.

We’ll likely see more such caution. The EPA issued a memo last year promising to increase audits of reported data and to test more cars itself. The message is clear: Generate reproducible test results or prepare to make adjustments—and headlines.