Loopholes and Fuzzy Math: The Tangled Science of MPG Ratings
In the wake of the revelations that Hyundai's and Kia's EPA mileage ratings were overstated, it's more important than ever for consumers to understand that mileage ratings are not always what they seem.
On every new-car window sticker, shoppers will see in the same fine print: "Actual results will vary for many reasons, including driving conditions and how you drive and maintain your vehicle."
That makes it sound like your gas mileage is solely your fault, and often, it is. But there's at least one other factor: The particular version of the car you buy and the features that come with it will affect the mileage of your new car no matter how passively you drive it.
Automakers determine gas-mileage ratings based on tailpipe-emissions tests, and the EPA admits that it audits only about 15% of automakers' claims for accuracy. Not every variation of every car model is tested by automakers, though, and you could easily wind up with a version that gets significantly worse mileage than what its Monroney sticker claims. Those claims take into account the engine, transmission and drivetrain, but there are many more variables.
What do we mean? Think sport trim levels, gigantic sunroofs, larger wheels, different seating configurations in large crossovers, optional storage panels ... you name it. They can all affect your gas mileage.
For his part, Bender bought a base V-6 Mustang with only a few options. He could have tacked on plenty more, including three available wheels and tires, two rear axle ratios and no shortage of spoilers, scoops, diffusers and louvers. Experts say all of those features can influence gas mileage, in some cases significantly.
Why don't automakers and the EPA offer car shoppers more explicit guidance?
Loopholes and fuzzy math, that's why.
Lots of Cars are Rated
While the EPA certifies gas mileage for plenty of cars and their various trims — 1,294 for the 2012 model year, to be exact — they leave most of the testing up to automakers.
What the EPA is really looking for is the version of a car that represents what most shoppers buy most often. Because of that, an automaker is only required to test "strictly the highest-selling" version, the EPA said. But if the automaker projects that the "high-selling" version will account for 40% of sales and two other packages will split the remaining 60%, it can take its mileage ratings for the car from that 40% package — even though more car shoppers buy the other two versions. The automaker could average the ratings for all three, which is voluntary, and it's unclear how many automakers do that.
What automakers can do, though, is give "subconfigurations," also known as those smaller packages, the same EPA mileage label without any additional testing, as long as they fall within a 250-pound test-weight window (meaning no more than 125 pounds over or under the weight of the tested car).
How does that play out? Let's say that a given car has both four-cylinder and V-6 engines. At a minimum, the vehicle could get two EPA mileage ratings. Let's say the most popular version, a four-cylinder, gets a combined 30 mpg rating. If the automaker creates a version that gets markedly better mileage, it often will break out different mileage ratings for that version. Examples include the Dodge Dart Aero and Chevrolet Cruze Eco.
On the flip side, let's say the automaker also makes a version that comes with a V-6 Sport Package that increases performance without changing the transmission or horsepower, breaking the 250-pound weight-change limit or becoming the most popular trim. Even though it's clear that those changes will affect mileage, the automaker does not have to publish new EPA mileage ratings for the Sport version. If you end up buying that version, watch out.
We reached out to several of the largest automakers to discuss EPA ratings. GM and Ford declined to comment.
Case Study: Toyota Avalon
In one current example, Toyota engineers are not even testing the newest Avalon's mileage ratings; instead they're relying on specs drawn from the Toyota Camry V-6 and the Lexus ES 350, said Randy Stephens, a Toyota engineer.
"For Avalon, for example, we have for the V-6 model two fuel-economy ratings on the label. One is based on a 17-inch tire model, and one is based on an 18-inch tire model. The only reason to separate those out is because of the test weight. So as vehicles move into the different test weight categories for emissions and fuel economy, they're certified separately," Stephens said.
"As long as [the Camry and the Avalon] have the same engine, powertrain and test weight class, then we can certify them together," he said. "You certify just one of them, in this case the Camry, and then by paper calculation you just have to show the EPA that there is no significant difference to fuel economy that would push you out of that range, and then they would accept that.
"It really is a true paper calculation, but the cars are just spec-wise so close to each other so there's really no fear that there is a difference," he said. "In Avalon's case, for example, we are marginally heavier than the Camry, but we have better aerodynamics. If we can show on paper that all these factors work out ... then we are totally fine."
The difference in weight between a Camry XLE V-6 and an Avalon XLE with a standard V-6 engine is just 66 pounds despite the Avalon being six inches longer.
Some Features Matter
Are there drastic differences with popular features? Not in most cases. Most fall within the EPA's mileage window.
"As long as the engine and the type of transmission are the same, it's within the same range," said Andrew Smart, who directs industry relations at the Society of Automotive Engineers. "With or without a sunroof, it makes not a significant enough difference.
"[If you] looked in the overall range that the EPA gave you, you might find that one [version] came in at the bottom and the other came in at the top," Smart said. "They've managed to have two extremes, but yet the lighter one manages to have the same fuel economy [rating] as the heavier one."
But those variations can matter to real-world mileage, if not the official rating. Rolling resistance on tires can alter fuel efficiency by as much as 4% either way, Smart said, and every 100 pounds added or dropped affects efficiency by as much as 2%. Throw in a sunroof, power-sliding doors and a power tailgate and a model like the V-6 Toyota Sienna minivan can see its weight increase by as much as 205 pounds. But all front-drive versions of the minivan get the same EPA rating.
Equipment packages can affect mileage, but Toyota engineer Stephens said it's up to automakers to calculate how much a certain package could affect gas mileage.
"Within those test weights, if we were to change tires or change aerodynamics or something like that, we would kind of have to, on the honor system, self-calculate how much that would affect fuel economy," he said.
But automakers don't have to. Rating every single subconfiguration would require a huge number of tests.
"Nobody has the resources to test every single vehicle combination — it would be thousands of emissions tests," said Robert Bienenfeld, Honda's senior manager of environment and energy strategy. "If you get the heaviest version of that vehicle or the lightest version of that vehicle, there's a range of fuel economy that you'll get.
"The intent is to be in the fat part of the bell curve, where the vast majority of people are very close to that," he said.
'Drive it Right'
Bender, who connects with other Mustang owners, said even the sportiest versions should still achieve window-sticker gas mileage.
"Even with the optional axle ratios, you're able to achieve the EPA numbers if you drive it right," Bender said. "Even the guys with the GTs with the 3.73 gear — which is the deepest that you can go — if they drive them easy, they get the EPA number."
But it isn't the in-the-know Mustang owner automakers should worry about. It's the shoppers who spend more to get those stunning 20-inch wheels with a luxury package adding power liftgates, power-folding seats and panoramic sunroof who are then dumbstruck at the gas pump. Where will their frustration be targeted?