Your mileage may vary.

We've heard it for years, we all know it's true, and yet we put our trust in the EPA's fuel-efficiency ratings as a guide to what kind of gas mileage a car will really get.

Now, two cars--the 2013 Ford C-Max Hybrid and the 2013 Ford Fusion Hybrid--have brought the failings of the EPA test system to the fore.

10-15 percent leeway

In general, buyers seem comfortable with variance of 10 or 15 percent from the advertised EPA ratings.

And although automakers generally publicize only the higher rating number (for gasoline cars, always the highway cycle), the EPA's combined rating is usually pretty close to real-world fuel economy for most buyers--within that margin.

But the new 2013 C-Max and Fusion hybrid models have generated a drumbeat of dissatisfied buyers, who claim their real-world mileage doesn't even come close.

Inevitably, there are now lawsuits.

'What was Ford thinking?'

Here's just one of many comments Green Car Reports has received on the topic:

OPEN LETTER TO FORD: I thought my 2013 C-Max would be a Prius killer. NOT! As a returning Ford buyer, I feel deceived. I want to support U.S. companies and U.S. jobs.

What was Ford thinking when they published 47/47/47 estimates? Based on the advertised EPA estimates, I would have been OK with low 40s, but 28-33 mpg is not even in the ballpark.

This is not an issue about EPA testing standards, but rather an issue about setting false customer expectations in order to promote sales.

Ford's "47MPG" marketing campaign tarnished what should have been the rollout of a truly remarkable vehicle, the C-Max. Real-world MPG estimates should have been promoted in the mid-30s.

2013 Ford Fusion Hybrid

2013 Ford Fusion Hybrid

Only EPA estimates allowed

We've gotten dozens of comments like this, following our coverage of the disparity between published 47-mpg combined EPA ratings and real-world figures achieved by drivers.

There's just one problem: The writer above is wrong, in that this IS actually an issue about EPA testing standards

That's because the only fuel-efficiency figures any carmaker may use in their advertising are those derived from the standardized EPA testing cycles.

Carmakers can commission independent studies--VW did just that to show its VW Jetta TDI diesel outperforms its EPA ratings--but they can't quote those results in their ads.

And that brings us back to the question that Ford has highlighted: Are the EPA test cycles flawed?

EPA mostly on target

The answer seems to be that for most cars, they're essentially on target.

They were last changed in 2007, to reflect real-world results achieved by owners of Toyota Prius and Honda Civic Hybrid vehicles.

2013 Ford C-Max Hybrid, Catskill Mountains, NY, Oct 2012

2013 Ford C-Max Hybrid, Catskill Mountains, NY, Oct 2012

Those vehicles had previously been rated with mileages above 50 mpg, but the EPA tweaked its "adjustment factors" for hybrid cars and brought the numbers down.

The 2007 Toyota Prius, for instance, went from a highway rating of 60 mpg to 48 mpg.

From two to five test cycles

Almost all 2013 cars are now tested using five different cycles: the old EPA city and highway cycles, plus a "cold soak" version of the city cycle with the heater operating.

The two new and somewhat tougher tests that have been added are known as 5C03 and US06 (and not worth explaining in detail here).

The issue this year seems to be that Ford has built a hybrid system with powerful enough electric motor-generators that their C-Max and 2013 Fusion hybrids simply stay in electric mode far longer during the test cycles than in real-world driving.

And this is undoubtedly exacerbated by the fact that the Fords offer punchy acceleration--much faster than any Prius--which tempts drivers to use it, which severely lowers fuel economy compared to the test cycles.

Huge effort

Changing the EPA test cycles--tossing out the 30-year-old procedures and starting from scratch--would be a huge effort.

Proponents say that cars have changed so much, and their performance has gotten so much quicker, that today's real-world traffic is nothing like the traffic conditions that prevailed in the late 1970s.

The national speed limit back then was 55 mph, by the way. When's the last time you drove 55 mph on a freeway, hmmmmm?

And it might not allow apples-to-apples comparisons between cars of different model years.

Our bet: adjustment factors

On the other hand, the National Highway Traffic Safety Administration (NHTSA) and the Insurance Institute for Highways Safety (IIHS) both routinely make their crash-safety tests tougher, and simply explain it in their ratings using footnotes.

If we had to guess, we'd expect the EPA to study the Ford case--as they're already doing--and then further tweak their "adjustment factors" to bring their mileage into line with reality.

What do you think the EPA should do? Leave us your thoughts in the Comments below.


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