On Thursday, Ford Motor Company announced it is reducing the fuel economy label on its C-Max hybrid.
Ford's initial rating for the C-Max -- much vaunted in advertisements -- was 47 miles per gallon, in combined city and highway driving.
Now, the sticker will read 43.
Other automakers hope the potential embarrassment for Ford doesn't mean more problems for them. Calculating fuel economy is complex and expensive enough as it is.
Just how do they get that number?
The long road to that MPG sticker in the dealer showroom starts at companies like SAKOR Technologies in Okemos, Michigan, which makes an emissions and fuel economy measuring device called a dynamometer.
The U.S. Environmental Protection Agency requires emissions testing on just about everything that combusts fuel, from weed whackers to the building-sized engines used by oil and gas pipelines. SAKOR makes dynamometers -- "dyno" for short -- for them all, including ones that test hybrid engines, along with their battery systems.
The hybrid dyno is off-line on the day I visit SAKOR, so President Randy Beattie hooks up a weed whacker to the smallest dyno his company makes.
"Notice it automatically started the engine," Beattie points out as the engine starts to whine. "Now it will go up to speed, warm itself up, and it'll get to a point where it's changing speed and torque levels continuously."
To test a car, drivers take new vehicles through a road test. It involves accelerating to 60 miles per hour, and then coasting down a hill to zero. That "coastdown" test captures vehicle-specific data including wind and tire rolling resistance.
Once that data is entered into the dyno, the vehicle, or its engine, is hooked up to the device.
The EPA sets the parameters for the tests. Every car goes through the same tests so customers can get an "apples to apples" comparison among vehicles.
There are five tests: one for city driving, with its typical lower speeds, starts and stops, and frequent braking. There's a higher-speed highway cycle, which includes some slowing down and braking; one for cold starts; one for high temperatures; and one for aggressive driving.
Many automakers use that five-cycle test, but the EPA also allows companies to use the older two-cycle test, with a city and highway cycle. A "discount" calculation factors in the other three cycles.
The stakes are high for everyone
Beattie says auto companies spend hundreds of millions of dollars conducting fuel economy tests. Labs where the testing is done must strictly control the temperature, humidity, and even barometric pressure of the air going into the engine, because all these factors affect fuel economy.
"It's very high stakes, and the data better be good," says Beattie.
The stakes get higher each year, as the government's new fuel economy regulations go into effect. Those Corporate Average Fuel Economy (CAFE) standards will require greater fuel economy from cars than ever before.
Improving the fuel economy of an already highly fuel efficient car may not be rocket science, but it's darn close.
"It's getting more and more difficult to meet these numbers," says Beattie, "because (automakers) have already thrifted out all of the things they can do technologically that are easy to get more mileage, and now they're having to get really sophisticated."
Ford's C-Max MPG shortfall isn't the first for a hybrid - just the biggest
Ford isn't the first automaker to run into trouble with the EPA MPG sticker on a hybrid.
Customers who bought the first generation of the Toyota Prius complained they didn't get the fuel economy they expected. And that was before fuel economy was on every customer's mind.
Honda just settled a class action lawsuit over claims customers could not achieve the MPG on a previous generation of the Civic hybrid.
For the current generation of the companies' hybrids, there's less disparity. The EPA sticker for the 2013 Prius is 50; customers on www.fueleconomy.gov report an average of 47 miles per gallon.
Many customers of the current generation of the Civic hybrid say they're beating the EPA estimate.
Robert Bienenfeld is Assistant Vice President of Environment and Energy Strategy at American Honda.
"People paid extra for that hybrid," says Bienenfeld. So if they are five to ten percent short in their fuel economy number, they're bound to be disappointed.
But in some instances, people only start paying close attention to their fuel economy when they buy a hybrid. "What they may not realize," he says, "is maybe, if they are driving in the same way, they were probably ten percent short in their last car."
Even so, Bienenfeld says it's important to meet customers' expectations.
Honda has a separate set of engineers whose responsibility is to verify the fuel economy number given to them by the engineers doing the fuel economy tests.
"If the certification group doesn't get the same number that the development team does, then, that's just too bad for the development team, and they miss their target," says Bienenfeld.
How Ford got it so wrong
Most automakers believe the EPA's five-cycle test works pretty well at giving a reliable average for fuel economy that will hold up in the real world.
As long as the engineers don't cheat, that is. Hyundai had to recently reduce the MPG sticker by a few miles per gallon for many of its vehicles, after the EPA discovered the data for the coastdown portion of the tests was "creatively rounded," as one person wryly noted.
But most people, including Consumer Reports, are reporting 37-39 miles per gallon in the C-Max, a far cry from 47. That disparity has many in the industry scratching their heads.
Did Ford game the system in some way, some wondered? Did Ford engineers design the C-Max specifically to do well on the EPA test?
Ford, and the U.S. EPA, say no.
But in retrospect, it appears Ford made two wrong choices in arriving at the C-Max fuel economy number.
First, Ford took the fuel economy number for the Fusion hybrid, and applied it to the C-Max.
The EPA permits that, as long as the vehicles are the same weight and use the same powertrain, and as long as the vehicle that is actually tested is the higher-volume seller.
That tends to work out fine for most cars. But "this did not work for the Ford C-Max," says Chris Grundler, Director of the Office of Transportation and Air Quality for the U.S. EPA.
Grundler says small differences in design between very fuel-efficient vehicles like the Fusion and C-Max can add up to big differences in fuel economy.
"In this case, (the C-Max) is less aerodynamic than the Fusion," says Grundler. "It has a different wheelbase and tire, and it made a big difference in the result."
Ford also used the discounted two-cycle test when it tested the Fusion hybrid.
In subsequent tests of the C-Max, engineers collected data on the C-Max from the coastdown test.
And, they used the five-cycle test when they hooked the C-Max up to a dynamometer.
Ford could face closer scrutiny from the EPA on future vehicles. The agency would not confirm if it had put the automaker on a so-called "100% confirmatory" list, meaning the fuel economy on every vehicle made by the company will be double-checked by the government for a time.
Ford says it will send a check for $550 to each of the 32,000 people who bought C-Maxes. The company also faces several lawsuits seeking class action status filed by disappointed customers.
Anthony Montisano hasn't joined one of those lawsuits, yet. But he is disappointed.
Montisano is a San Diego attorney with a secondary office in a city many miles away. Hoping to save fuel costs on that long commute, and smitten by the C-Max's design and comfort, he bought a C-Max in February.
He believed the ads, and the salesman's reassurance, that his C-Max would get 47 miles per gallon.
"And ... no. It doesn't."
Montisanto is getting between 35 and 37 miles in the vehicle - probably because much of his driving is on the highway, where the battery system isn't much use.
He says he's not happy with Ford, and he figures a lot of other people probably aren't, either.
"It's not going to sit right with your consumer," he says of the issue. "And it's going to leave a bad taste for all future purchases by those customers of Ford vehicles."
For its part, the EPA is planning a review of its fuel economy standards, to make sure they give as accurate an MPG estimate for hybrid vehicles as for non-hybrids.
"We're going to see more (hybrid vehicles) in the future, which is why we want to nip this in the bud," says the EPA's Chris Grundler.
Other automakers remain concerned. They hope Ford's C-Max issue doesn't throw a monkey-wrench into a system that overall, works pretty well to give customers a reliable number for average fuel economy.