“Claimed” fuel economy is a lot like Santa. We all like to believe in both, but few of us have ever actually seen either and nobody believes those who say they have.

Modern, fuel-efficient cars are supposed to reach average economy figures in the 1960s and 1970s from their super-advanced engines, but there seems to be a constant undercurrent of public grumbling about how nobody can get anywhere near the claims.

In the US it seems to be different. At the Los Angeles Auto Show in November, Volkswagen’s North American wing was shouting very loudly about how the Golf ‘Clean Diesel’ can do a whopping 42 mpg on the highway – embarrassing by EU standards. Are cars made for America just rubbish, or is there something else to it?

To find out let’s first level the playing field. US gallons are smaller than the mighty British ones; 3.78 litres versus 4.54 litres. But even making the necessary adjustments, the American Golf still only musters 50.4 mpg in UK terms. And remember, that is at a cruise. They don’t measure any kind of ‘combined average’ figure.

But things aren’t always as they seem, because Americans don’t like diesels and to a nation that is used to mpg figures in the 20s, a car that returns over 40 is a revelation.

They’re not ready to buy something as environmentally focused as a Golf BlueMotion yet, and their Clean Diesel is in fact just a 2.0-litre TDI, roughly similar to a mid-range European Golf that achieves 58.9 mpg on the well-known and much-maligned combined cycle – 68.9 mpg on the extra-urban one.

This American Golf is a model that doesn’t even have engine stop/start technology – outside of hybrids the idea hasn’t really taken off yet across the pond. The idea of a 74 mpg family hatchback like the Golf BlueMotion is about as real-world as Wile E. Coyote.

And there are differences in the actual fuel economy tests themselves. The current European system is designed to squeeze every last drop of potential forward motion out of economy-tuned engines. It’s even daft enough to allow up to 50 seconds for acceleration up to 62 mph.

The US system is designed to reflect more real-world driving, including using the AC, cold start cycles and other factors that will undoubtedly reduce fuel economy.

The end result is a figure that might well be more typically attainable than the EU figure. Rory Lumsdon, Manager of Product Affairs at Volkswagen UK, said: “In a way, the US [test] is a little bit more real world than the EU one.”

The old problem of poor quality US fuel is a thing of the past, though. All US diesel is, since December 2010, Ultra-Low-Sulphur Diesel, as distinct from Low-Sulphur Diesel.” “So the type of diesel fuel is no longer an issue. This means that the different MPG figures are most likely to end up being down to the different test cycles used in calculating MPG.”

The dilemma is that big fuel economy numbers sell cars, but the higher the expectation, the bigger the perceived problem is when the car just won’t match up.

Manufacturers themselves don’t actually claim anything, or at least not in the first instance. The testing is done by independent organisations and then the resulting EU-certified figures are legally binding – companies have to use them whether they like it or not.

They then have to decide whether or not to put their weight behind the published figures and effectively take ownership of them, offsetting commercial gain from sales and brand image with any loss of reputation from their cars not matching what essentially amounts to a theoretical, and therefore always elusive, maximum.

It becomes doubly awkward for plug-in hybrid vehicles like the Chevrolet Volt, which officially recorded 232.5 mpg on the European combined cycle.

Craig Cheetham, PR Manager at Chevrolet UK, acknowledges it’s an awkward situation. “It’s somewhat frustrating as customers often think we’re claiming a figure and then blame the manufacturer if they can’t achieve it,” he said.

“It’s particularly confusing with cars like the Volt, where the cycle of 100km means the car will complete most of the test in full electric mode, but not all – which gives us an MPG figure of 232.5 mpg on the European cycle and 93mpg on the US cycle, neither of which is truly representative of what you might achieve in the real world, as there are so many variables in the way people use the car.”

Some cars, however, can be driven way in excess of their fuel economy figures. They tend to be large, thirsty cars like the Vauxhall VXR8. At the 2011 MPG Marathon in the Cotswolds, the V8 muscle car achieved 32.14 mpg; 53% more than its published average of 21.

Car buyers are getting increasingly frustrated with what they see as unrealistic fuel economy figures. Perhaps lowering them would be good for the industry as a whole, and the European Parliament is looking at changing the system to do just that.

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