Here's why real-world mpg doesn't match EPA ratings



Has this happened to you? You go shopping for a new car and great fuel economy is high on the list of things you want. You buy a car that's rated 30 mpg on the highway and 28 mpg overall by the Environmental Protection Agency (EPA). But after a month or so of driving around, you find that the best fuel efficiency you can get is a measly 24 mpg average. You might acknowledge that you drive a little faster than the speed limit, but you're no hot-rodder.

So why doesn't your fuel economy match the EPA rating?

This question comes up most often when gasoline prices are rising. It is especially pertinent now as a fuel-efficiency ratings scandal sullies Hyundai Motor Corp.'s image and leaves millions of consumers wondering if they can trust the EPA window sticker the next time they shop for a new car or truck.

Hyundai has had to roll back its fuel-efficiency claims for eight models, along with an additional five rollbacks for cars and crossovers sold by its sister company, Kia Motors. Hyundai and Kia issued a joint statement saying they will give customers a personalized debit card that will reimburse them for their difference in the EPA combined fuel economy rating, based on the fuel price in their area and their own actual miles driven. The carmakers explained that the issue was rooted in an error by its South Korean test crew. The problem has been corrected and won't happen again, Kia and Hyundai say.

That's good, and the fact that the inflated Hyundai and Kia fuel economy figures were caught and ordered corrected by the EPA is even better. It proves that the agency's system works.

But it took the EPA two years to act after fielding hundreds of complaints from Hyundai customers that the mileage they were experiencing wasn't even coming close to the official EPA estimates posted on the vehicles when they were on the dealers' lots.

For many, the explanation of what happened was an eye-opening look at how the system works: The EPA establishes the tests that yield the fuel economy figures, but for the most part it doesn't conduct the tests itself. It doesn't have the budget, equipment or manpower to test the hundreds of individual models with unique engine and transmission combinations that automakers produce each year.

Instead, the agency gives its test protocols to the auto companies and lets each test its own cars and trucks. It accepts as true the "EPA estimated" fuel-efficiency numbers each car company submits. To keep the industry honest, the agency runs scores of spot checks each year. Until the Hyundai/Kia mess, only two rollbacks had been ordered in the past decade, each for a single model. The 13 Hyundai and Kia models with overstated fuel efficiency represent an unprecedented breach that has some consumer advocates calling for the agency to conduct an industry-wide verification of fuel-efficiency claims.

Even if they don't own Hyundai or Kia vehicles, many car and truck owners are stewing over the gap between the fuel efficiency they've been promised in advertising and window stickers and what they're actually getting on the road. Honda, for example, continues to struggle with customer dissatisfaction with the fuel economy of older model gas-electric Civic Hybrid sedans.

Events may prove that there have been deliberate efforts by one or more automakers to game the system and achieve higher testing scores than warranted under the very specific, federally mandated testing cycles. But for the most part, the fuel-economy gap exists for a more mundane reason: Real people drive real cars in the real world. There are so many variables that the idea of an absolutely accurate rating of average mpg is laughable. But to new-car buyers, it often feels as if the joke is on them.

City vs. Highway

A key element in assessing the EPA rating for a vehicle's average fuel economy (EPA combined) is the split between highway and city driving. Almost all cars and trucks deliver better fuel economy while cruising at 55 mph on the open highway than they do while stopping and starting at low speed on city streets.

The EPA rating for combined mpg presumes that we drive 55 percent of the time in the city and 45 percent of the time on the highway. Most people simply assume that's the case in their own driving. But many motorists — especially those in urban regions with lots of traffic congestion — spend far more time driving in city conditions than they do on the open road.

It's also useful to remember that even when a lot of driving time is spent on a highway or freeway, it only counts as highway driving when the average speed is 50 mph or so. Crawling toward Manhattan on the Staten Island Expressway at 10 mph is actually city driving, even though it's technically done on a highway.

If you diligently keep track of the number of miles that you are really driving at highway speeds each day for a month or so, you might find that you really are doing most of your driving at less efficient city speeds. If you have a newer vehicle with a trip meter that displays your average speed, keeping track is easy. Just set it to zero each morning, and then check each evening for your actual average speed for the day.

The Hybrid and EV Exception

Just to further confuse the issue of fuel-efficiency ratings, the city-worse/highway-better driving pattern doesn't hold up when it comes to hybrids and electric-drive vehicles. Such vehicles use an electrical system to stop and start a gas-powered engine or to deliver electric-only propulsion, which improves fuel efficiency at low speed.

The EPA rating procedure for plug-in hybrids and battery-electrics is new and likely will need some additional fine-tuning. An example is the 2012 Fisker Karma. The manufacturer's tests show that the car will reliably deliver 50 miles of range in all-electric mode if it starts out with a fully charged battery. (Once the battery charge is depleted, a gasoline-powered four-cylinder engine-generator kicks in to continue providing power for the car's dual electric motors.)

So Fisker was stunned when the EPA released its rating and said the Karma is only good for 32 miles of range in all-electric mode. To see which estimate was closest to reality, ran a two-day Fisker Karma fuel-efficiency test. Even when climbing hills and plowing down the freeway at speeds well in excess of 70 mph, the Karma was always able to do better than the EPA estimate. It averaged 40.8 miles of range in all-electric mode over two days. In a city loop with no highway driving, it attained 45.4 miles of range.

Surprisingly Close Fuel Ratings

What may be surprising to many is the fact that ever since the EPA modified its testing process and applied those changes to the 2008 model year, its ratings aren't really all that far off the real-world mpg that consumers get. While there are lots of people who cannot under any circumstances get their vehicles to come close to the official ratings, there are also lots who regularly meet or exceed them.

A special "Your MPG" section of the Department of Energy's Fuel Economy Web site lets people report their own fuel economy averages. No one double-checks the figures, and those reporting might be bragging about good fuel economy. But the self-reported mpg averages for various vehicles, which the site has been compiling for several years, tends to show better fuel economy than the EPA ratings. keeps close track of fuel economy in its fleet of long-term cars and trucks and has found that most come within 2 mpg of the EPA combined average. And that's with about 20 different drivers subjecting each vehicle to a wide variety of driving styles.

Dan Edmunds,'s director of vehicle testing, says that the cars and trucks that are most likely to significantly fall short of their EPA combined average ratings are those that are underpowered — a big SUV with the optional, downsized four-cylinder engine instead of the standard V6, for example. One exception — now explained by Hyundai's admission that its fuel efficiency numbers often haven't been correct — is that Edmunds testers were unable to get the Hyundai Elantra to live up to its EPA highway rating, even though the car is not underpowered. A 2011 Hyundai Elantra was part of "fuel sipper" test in 2011 and the results revealed that the Elantra consistently underperformed in highway fuel economy.

No Bias

Fuel-efficiency researchers David Greene and Zhenhong Lin at Oak Ridge National Laboratory in Tennessee recently compared EPA estimates and "Your MPG" reports and found no evidence of over-performance bias in the government's numbers. But what the researchers did identify were six factors they said could cause a vehicle's real-world fuel economy to vary significantly from its EPA rating.

At the top of the list is how much city driving the car does. The proportion of stop-and-go driving could reduce the EPA combined city/highway fuel efficiency for a particular vehicle by as much as 27 percent, they say. Individual driving style can cause the second biggest variation, lowering fuel efficiency by up to 18 percent. Edmunds testing supports these conclusions. "Calm" drivers, those motorists who don't accelerate constantly and who avoid unnecessary lane changes, get 35 percent better fuel economy than other drivers.

Other factors in play are air-conditioner use (up to 14 percent reduction in fuel efficiency), vehicle size (up to 15 percent reduction) and the region in which the vehicle is primarily operated (up to 12 percent less fuel efficiency, because hot weather and mountainous conditions take a toll on fuel economy).

Fuel type also affects mileage. Most gasoline in the U.S. today is 8-10 percent ethanol, but the EPA does its tests with 100 percent gasoline in the tank. The use of ethanol to increase the amount of oxygen in gasoline for better combustion can reduce fuel efficiency by around 2 percent all by itself.

Be Honest About Your Driving

The point here is that the common advertising disclaimer of "your mileage will vary" is something every car shopper must take seriously. Honestly assessing your specific situation is the only way to adjust the EPA ratings down — or up — to more accurately reflect the fuel economy you can expect to see once the car or truck is yours.

One way to do a reality check on a car's EPA rating is to deduct 10 percent from it if you consider yourself an aggressive driver. This means someone who routinely exceeds speed limits and hurries away from stop signs and red lights. You can stick with the EPA rating if you consider yourself an efficient driver. You'd need to reevaluate that self-description if you find that your car or truck is consistently failing to achieve the rated combined average, however.

Real-world fuel economy can also be diminished if you use the air-conditioning a lot, haul heavy loads of cargo or passengers, or even live at the top of a hill.

The Ratings Are Here To Stay

If the EPA ratings aren't a good reflection of actual fuel-efficiency performance, can we expect to see them changed any time soon? The quick answer is no. There's not a lot of will on Capitol Hill to change the way the EPA figures fuel efficiency for passenger vehicles.

In government and policy circles, it's well known that the EPA's fuel economy figures are too optimistic, but changing the system to more closely reflect real-world averages would mean lowering the bar on all the measurements of the current state of fuel efficiency on the American highway, and no one is eager to admit that we really haven't come as far in fuel-efficiency gains as it seems.

The 54.5-mpg average for fuel efficiency among new vehicles that the Obama administration is proposing for 2025 will really be somewhere down around 36-38 mpg in real-world terms.

The MPG Testing Gap

The problem is that the federal government uses two different procedures to compute the fuel-efficiency numbers it provides to consumers. First, there's the original procedure for the Corporate Average Fuel Economy (CAFE) values that is used for regulatory purposes. Introduced in 1975, it is based on standardized highway and city driving cycles that are replicated in a computer and measured on a dynamometer.

But because people complained of a big gap between real-world fuel economy and the test values with this original CAFE test, Congress asked the EPA to make some modifications to the test, which it did in 1985. Unfortunately, the changes in this second-generation test apply only to the window sticker that consumers see for individual vehicles. Also, Congress didn't tell the EPA to modify the test for CAFE purposes.

As a result, the new CAFE measurement — the 54.5-mpg figure that's been in the headlines — is based on the original, congressional-endorsed test that was mandated in 1975. Meanwhile, the revised, EPA-endorsed fuel economy test is the one reflected in the ratings posted on the window sticker in all new cars and trucks. The EPA rating is also the one used in automotive advertising, most car-buying guides and online car shopping and information sites, including

And even though the EPA's test system was modified in 1984 and 2008 to try to make it more relevant, it still doesn't precisely reflect real-world driving. The EPA still uses stationary laboratory tests run on a dynamometer, which ensures repeatability but not necessarily the driving experience of consumers.

Wanted: Personalized MPG Estimates

Before 2008, the EPA tests overestimated fuel efficiency quite significantly. They didn't account for the fact that real-world driving tends to be more aggressive, faster on the highway and subject to more stops, starts and more rapid acceleration and deceleration events than the defaults built into the EPA test protocols.

The 2008 test update tried to correct for those factors and — despite complaints — it does seem to have brought EPA and real-world fuel economy averages a lot closer together than ever before.

So the issue now is not a rosy bias in the EPA testing results, says David Greene, the researcher at the Oak Ridge National Laboratory. As Greene says, "What we need are ways to predict an individual's fuel economy."

In the meantime, take any claims of fuel economy with more than a few grains of salt.

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