Back in December, our pals at the not-so-secret above-ground Consumer Reports auto-testing facility called into question the 47 miles per gallon (highway and city combined) number touted by Ford for its C-Max Hybrid, saying their tests showed a still-respectable but lower-than-advertised 37 mpg. Now, only a mere eight months later, the EPA is also saying that number should be lower.

The EPA announced this afternoon that it is revising that highway and city combined estimate for the C-Max Hybrid to 43 mpg (45 mpg city; 40 mpg highway). The previous estimates had calculated 47 mpg across the board.

While the mileage labels you see on most cars say “EPA estimated,” most of these estimates are provided to the EPA by the manufacturers, with the EPA only doing spot-checks on around 15% of new vehicles to corroborate the estimates.

After Consumer Reports published its findings, Ford maintained that it had tested the C-Max per EPA standards and that perhaps CR achieved different results because it was driving too fast or in weather that was too cold. To resolve the issue, the EPA got itself a C-Max (we don’t know if it purchased it at retail or if the car was furnished by Ford), broke it in for 4,000 miles and only then performed its own fuel-economy tests that showed results below what Ford had claimed, especially with regard to highway mileage.

So how did this happen? Back in 1977, the EPA decided that vehicles with the same engine, transmission and weight class could use the same data for their fuel economy labels, and this has not been a problem with traditionally gasoline powered engines. Thus Ford was able to take the data for its Fusion hybrid and use it on the C-Max.

Alas, because hybrids use significantly less fuel than traditional cars, they are significantly more sensitive to little quirks and differences in design that would not have impacted the mileage of conventional gasoline-powered vehicles.

“To date, most high-efficiency hybrids have been used in a single vehicle design and therefore do not have this issue,” explains the EPA in its statement. “The Ford hybrid family is one of two examples in the industry where advanced technology vehicles with the same engine, transmission and hybrid components are used across multiple vehicle designs.”

Ford will not be fighting the revised numbers — which honestly, are still pretty darn good compared to many vehicles — and will be slapping the new labels on C-Max voluntarily.

Additionally, Ford will reimburse C-Max owners $550 each for the extra fuel costs. C-Max lessees will receive $325 each.

Thing is, while issuing revised numbers for the C-Max hybrid resolves this particular issue, it doesn’t solve the long-term problem that consumers will face as manufacturers begin expanding their hybrid lines. If the auto industry doesn’t voluntarily begin doing individual testing on hybrid models — no matter how similar one model is to another — we’ll only see more inaccurately labeled vehicles roll off the line.

To that end, the EPA says it will be working with consumer advocates, environmental organizations and auto manufacturers, “to propose revised fuel economy labeling regulations to ensure that consumers are consistently given the accurate fuel economy information on which they have come to rely.”

For more of Consumer Reports’ coverage of the Ford hybrid mileage situation, check out ConsumerReports.org.

For C-Max owners and those just curious about this change, the EPA has put together a fact sheet [PDF] for your perusal.

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