The recent proliferation of things such as paddle shifters, various transmission modes, and more-responsive sport settings has made for interesting back-and-forth negotiations between the automakers and the EPA when it comes to the particulars of vehicle testing.
For emissions purposes, cars are certified in groups arranged by similar characteristics (engine, transmission, etc.), and the model or trim level with the highest emissions must be tested by the automaker. There’s no wiggle room. This version is generally the heaviest or has the most aerodynamic drag, and that vehicle is tested in any driver-selectable mode, such as a transmission sport mode, that would worsen emissions. But for fuel-economy purposes, many things are up for discussion; it’s the automaker’s task to convince the EPA how the cars are likely to be driven in consumers’ hands.

The EPA’s Wehrly describes a hypothetical, but typical, discussion where an automaker wants to get around activating a particular feature that degrades fuel economy during the testing. Luxury automaker: “We have this feature, but no one uses it.” EPA: “Then why do you have it?” Automaker: “Because Mercedes has it. If we don’t have it, it looks bad.”

We thought the powerful BMW M5 starts up using “just” 400 horsepower to save fuel (the driver can push a button to get the full 500 horsepower). But most gimmicky buttons are just that: The EPA tested the M5 in both 400- and 500-hp modes and found no difference in the amount of fuel used. The demands of the test cycles never call upon all the M5’s horses anyway. And the Honda Insight’s econ mode—activated by a dash button and claimed to improve fuel economy—registered no effect, either. It relaxes throttle response, so the test driver simply compensates with additional throttle to achieve the required speeds. However, GM’s skip-shift device, found in the Corvette among others, irritates by forcing the driver to shift the manual transmission from first gear to fourth at low speeds and was developed precisely to improve fuel economy on the test cycles. It was approved by the EPA, i.e., not considered cheating. As this is written, Porsche is in negotiations to report fuel economy with the Panamera’s standard, mileage-enhancing stop-start system in place, which switches off the engine as the car stops, then refires it in a flash the moment the driver’s foot releases the brake. (Surprisingly, the sports-car specialist will have the first nonhybrid start-stop system in the U.S.) But the EPA’s likely counterargument to Porsche using the higher numbers is that with the imported car, the system has to be manually switched back on each time the car is started (the opposite of European Panameras), which means drivers will likely forget to activate the system and let the engine continue to idle at stops. Porsche wouldn’t say how much of a fuel-economy boost the system provides, but 18 percent of the EPA city cycle is spent idling, where the Panamera could be saving fuel with its engine switched off.