The Truth About EPA City / Highway MPG Estimates

Where do those mileage figures come from?








So how does the EPA lab work?







In 1972, when regular gas was 35 cents a gallon, test cycles were invented by the newly created EPA to measure exhaust emissions. The first test cycle, which sought to mimic rush-hour traffic in downtown Los Angeles with an average speed of 21 mph, is called the FTP, or city cycle, and is still in use today. This dyno test is 11 miles long, takes just over 31 minutes to complete, involves 23 stops, reaches a top speed of 56 mph, and has maximum acceleration equivalent to a lazy, 18-second 0-to-60-mph run. A second cycle to measure highway driving was added in the late 1970s as part of the introduction of corporate average fuel-economy (CAFE) regulations. This 10.3-mile cycle with an average speed of a paltry 48 mph and acceleration no more severe than in the city test, may have been somewhat realistic in the days of the national 55-mph speed limit but doesn’t come close to approximating the manic highway behavior of today’s drivers. As a result, and even though the test figures were adjusted downward starting in the early 1980s in an attempt to produce more realistic sticker values (by 10 percent for the city test and 22 percent for the highway), the EPA numbers gave drivers too optimistic an expectation of fuel economy for decades.

In 1987, Congress allowed the states to increase highway speed limits from 55 to 65 mph, but it would take another 21 years for the EPA to adopt tests that provide more realistic projections. To accomplish this, the EPA added three additional test cycles to the original two for 2008 model-year cars.

These new tests—they had, in fact, been in use since the late 1990s but for emissions purposes—are the US06 high-speed (80 mph max) cycle; the SC03, or “A/C,” cycle, which is very similar to the city cycle but run in 95-degree heat with the vehicle’s air conditioning filching fuel; and the cold FTP test, which is exactly the same as the city cycle but run at a frigid 20 degrees. Until the 2012 model year, however, automakers may use results from the two old fuel-economy cycles and then plug those numbers into an elaborate equation developed by the EPA that approximates the new five-cycle procedure. For 2012, all fuel-economy figures will derive from the results of the 43.9 miles covered by the five tests.


While piloting a Honda Insight at the lab, we found the high-speed US06 cycle impressively aggressive. The cycle still averages only 48 mph but has bursts of acceleration (similar to a 7.0-second 0-to-60-mph time) that are more than twice as demanding as the original highway cycle and a top speed of 80 mph. To keep pace, the Insight required at least three-quarters throttle, which made us appreciate the realism of the new tests and wonder if some underpowered cars may have a tough time keeping up with their acceleration demands.

The imperfection of human drivers is a big problem with repeatability of the tests, and hybrid vehicles in particular are very sensitive to throttle inputs—which, in turn, control engine starting and stopping and the amount of battery assist. We’ve heard complaints that fuel-economy results for hybrids can vary by as much as 10 or 20 percent depending on the driver, although the EPA claims the results are repeatable within one to two percent.

Measuring fuel economy during the tests is likewise hugely complex, which is why the automakers and the EPA both follow precisely the same protocol. For openers, the chemical composition of fuel varies slightly, so simply retrieving it from a local gas station won’t produce repeatable results. The EPA has a specialized company manufacture small batches of consistent fuel, which is 93 octane (cars running 50-state certifications get a slightly different, 91-octane “California” blend). Before being used, the gas is analyzed to measure its properties, and fuel economy is then calculated based on the measured carbon content of the various tailpipe emissions—unburned hydrocarbons, carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), and oxides of nitrogen (NOx)—that are collected in bags made of a special Kynar plastic. A $350,000 gas-analyzing machine then makes minute measurements. The one-percent accuracy of this machine from Japanese company Horiba is amazing considering the minuscule amounts of some of the exhaust constituents—some in quantities as low as a half-dozen parts per million.

The final city and highway ratings are calculated by taking the fuel-economy results from specific portions of each of the five tests and piecing them together.


This fairly recent revision in the testing protocol isn’t cheap for automakers to adopt. The EPA estimates that to build a test facility capable of running one SC03 or cold FTP test per day would cost $10 million. Even the EPA’s own lab is incapable of performing two of these new tests, which it instead contracts out. But the revised tests are likely worth it as we’ve noticed a big change: Pre-2008 test cars in our enthusiastic hands rarely returned fuel economy anywhere near the EPA’s highway projection and would regularly get poorer mileage than even the city figure, which—not counting hybrids—is the lower of the two. Now we generally see mileage between the two numbers, and with a little restraint, the EPA’s highway number is usually within reach. The EPA maintains a repository of ratings from 1985 to the present at

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.

The EPA says its test results are almost always very close to the automakers’ numbers. When they don’t match, a long discussion ensues. According to the rules, if the EPA retests a vehicle to assess an automaker’s results, the fuel economy should be within three percent. If not, the automaker can choose to accept the lower of the two sets of numbers or request one additional retest. The EPA seems convinced that cheating is rare, given the complexities of the current tests and the high stakes involved in getting caught. After all, the EPA has to give its approval before an automaker is permitted to sell a particular model. If that’s not enough of a deterrent, there are also big fines for cheating. In 1998, all the major players in heavy-duty diesels (Caterpillar, Cummins, Detroit Diesel, Mack Trucks, Navistar, Renault, and Volvo) were cited for an engine-control strategy that leaned out the air-fuel ratio at steady highway speeds, which boosted fuel economy at the expense of NOx emissions. The EPA slapped those seven manufacturers with what it called the “largest civil penalty in environmental enforcement history,” a total of $83.4 million in fines.

One last fuel-economy tidbit: Don’t even think of comparing EPA figures with stand­ardized fuel-economy tests from other countries because the test cycles are very different. For example, the European highway rating, called “extra urban,” is higher than the EPA’s by about 30 percent, so a rating on that cycle of, say, 60 mpg, would be closer to 40 in this country. The mainstream press, not realizing the difference, often complains that automakers refuse to bring efficient models here when, in fact, they may not be all that efficient when measured by U.S. standards.

Next time you’re verifying fuel economy by topping off the tank and dividing the miles driven by the number of gallons, maybe you’ll appreciate the rabbit hole of complexity behind those numbers.


2011 Chevrolet Volt: 150 MPG?


How will the EPA measure the mileage of electric cars and plug-in hybrids? Particularly tricky are upcoming cars such as the Chevy Volt, which can travel a significant distance on electric-only power. If the EPA were to run its fuel-economy tests with a fully charged battery, the Volt would burn no gas at all. So would the miles per gallon on the city cycle read “infinite”? According to the EPA, a likely solution is to test this new breed of hybrid twice—once in electric-only mode and again with the battery drained and the gas engine providing electricity—and then combine the test results. But should the two tests simply be averaged? GM argues that most customers will plug in the car at night and therefore be running on batteries the majority of the time, so the results should reflect a significant portion of efficient, electric-only driving.

Pure electric cars, such as GM’s EV1 of the 1990s and, more currently, the Tesla Roadster, have fuel economy stated in units of kilowatt-hours (kWh) per 100 miles. For example, the Tesla gets 32 kWh per 100 miles city and 33 highway, but these figures are Greek to American car buyers accustomed to miles per gallon. Which is better, 100 mpg or 32 kWh/100 miles? Everyone involved, including the automakers, seems to agree that all cars, including electrics, need to be rated on a mpg-equivalent basis where, for example, the amount of energy used by an electric vehicle could be converted into a volume of gas with the same energy content. All window stickers would then contain mileage figures in mpg. The EPA is currently in long, soul-searching meetings to agree on a standard; a draft proposal should be submitted by year’s end. Whatever the specifics, GM expects the Volt to carry a fuel-economy rating of well over 100 mpg.


Lamborghini Murciélago: Buyers pay a $6,400 fine for the biggest fuel-sucking car in the nation.


When an automaker’s vehicle falls into “gas-guzzler” territory—below 22.5 mpg according to a different, “unadjusted” rating—or when its entire model lineup fails to meet the government-ordered fuel-economy average, somebody pays.

The gas-guzzler tax was created as part of the Energy Act of 1978 and is enforced by the Internal Revenue Service. The possible penalties for poor fuel economy range from $1000 to $7700 per vehicle, don’t apply to trucks, and are added to the sticker price. Since 1983, the IRS has collected nearly $800 million in these fees from new-car buyers.

It’s a confusing system that could only make sense to politicos because the fuel-economy numbers used for gas-guzzler calculations (referred to as “unadjusted”) aren’t the same as what appears on the window sticker. The unadjusted number is higher than the EPA’s combined mileage rating (a weighted average of the values posted on the window sticker: 55 percent city and 45 percent highway) by about 30 percent. Currently, the worst single fuel-economy offender is the manual-transmission Lamborghini Murciélago, which is rated at 8 mpg city and 13 highway (the Lambo’s combined score is 10 mpg and for gas-guzzler purposes gets an unadjusted 12.6 mpg). For this excess, the buyer pays a $6400 penalty. Again, because of rounding that happens behind the scenes, guessing the guzzler threshold based on window-sticker figures isn’t an exact science, but roughly, it begins at sticker ratings of 14 mpg city and 22 highway, such as those for a Mercedes-Benz S550. The Mercedes carries the minimum penalty, $1000.

On the other hand, automakers have to pay when they don’t meet the fleet-wide CAFE mandate that was also enacted in 1978 and is likewise based on the higher, unadjusted fuel-economy figures. For the 2009 model year, automakers must meet an average of 27.5 mpg for cars and 23.1 mpg for light trucks (below an 8500-pound gross vehicle weight rating) on a sales-weighted basis. Automakers’ fleets are further separated into three categories—domestic cars, imported cars, and light trucks—and each individual category has to meet the fuel-economy bogey. For each 0.1 mpg that an automaker misses this mark, it is fined $5.50 multiplied by the number of cars it sells. Conversely, if an automaker exceeds the CAFE requirements in a given year, it can carry forward a credit to offset future shortcomings. Currently being argued is whether companies should be allowed to buy and sell credits to each other. Electric-car maker Tesla, which has an inflated CAFE average of 244 mpg, thanks to an adjustment factor devised by the Energy Department, would certainly benefit. Meanwhile, Ferrari pays about $1 million per year in fines ($616 for each of its 1645 cars sold in 2008) in order to sell its breathtaking lineup here, while Mercedes’ fleet of imported cars came up 2.8 mpg short in 2007, which equated to a whopping, $29 million tab.

The Obama administration has recently announced plans to dramatically ramp up the CAFE thresholds to attain 39 mpg for cars and 30 mpg for trucks for 2016 (that’s average real-world fuel economy in the neighborhood of 27 mpg for cars and 21 for trucks) and to include vehicles up to a 10,000-pound gross vehicle weight. The policy also slices those average “car” and “light-truck” thresholds listed above into vehicle-specific fuel-economy targets that are based on the vehicle’s footprint (wheelbase times track width), so each model will, in fact, have to achieve slightly different fuel-economy numbers. Opponents of this modification argue that it will encourage production of larger vehicles because the larger the vehicle, the lower the fuel-economy hurdle. One thing’s for sure: These sweeping changes will significantly alter our vehicular landscape.

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