Realistically, as far as fuel consumption figures go, we all know that real-world numbers never get to manufacturer claimed figures, but there has been a great disparity in claimed and real-life mileage numbers for domestic offerings in Japan, enough to get consumers complaining.

All that will finally change with the full adoption of the JC08 chassis dynamometer test cycle for light vehicles. According to an Asahi Shimbun report, the JC08 standard of computing fuel efficiency – which was introduced in 2007 – is set to finally replace the country’s 10-15 mode cycle test as the de facto benchmark for claimed consumption figures.

The country’s transport ministry will require automakers to use JC08-based figures from April for their new models. As for current models, the figures will gradually be phased in by numbers based on the new testing method, by 2013, the report said.

For years, the mileage figures derived from the 10-15 mode test cycle has been the accepted norm. The only problem is that in real-life, consumers never got anywhere close to those numbers generated by the testing, with the variation from the catalogue figure being very noticeable.

The Toyota Prius, for example, is supposed to be able to achieve 38 km per litre according to the catalogue. In the real world, however, the hybrid manages an average of 21.7 km per litre, according to Japanese mobile phone site e-nenpi, which publishes mileage figures calculated on actual fuel and distance data provided by drivers. That’s a difference of 43%.

Part of the problem with 10-15 mode testing has been its lack of variation; though there are 25 tests in all, the vehicles go through their paces at a constant rate of acceleration and have to maintain the same speed on rolling road dynamometers at government testing facilities, not exactly reflective of real-world accuracy. The 10-15 running pattern begins with a warm start, lasts for 660 seconds and runs at speeds up to 70 kph.

It doesn’t help when transport ministry officials say that automakers adjust the programming of engine control systems to ensure good results and employ special test drivers for their vehicles.

With JC08, evaluation tests will measure complicated acceleration and deceleration maneouvres repeatedly in an effort to emulate driving in urban conditions. The pattern runs longer, at 1200 seconds, with both cold and warm start measurements, and the top speed goes up to 82 kph. In general, the figures from JC08 are expected to be 15% percent lower than that obtained through 10-15. but will reflect more realistic numbers.

With the UN moving towards the introduction of common international standards for different fuel efficiency testing methods by 2013, expect more in the way of ratification and standardisation as it goes along.