Why manufacturers' CO2 figures are a complete farce...
Have you ever wondered who calculates the CO2 figures for car manufacturers? Well, guess what, they do their own testing...
There is a specified test cycle, but there are no pre-conditions for hybrids, so they can arrive at the test with a fully charged battery, and finish it with an empty battery. The test route is only 11km long, and is carried out on rollers,... so wind resistance and body mass is not taken into account (very handy for Range Rovers etc...). By using non-representative levels of battery assistance, not having to accelerate a large mass (rollers, remember) and avoiding wind resistance, hybrids like the recently-trumpeted Range Rover's 2.5 tonne 89g/km hybrid can achieve incredible, but completely unrealistic, figures. And then the government believes them, and people in small, more efficient cars (in the real world) end up paying more road tax.
http://www.vcacarfueldata.org.uk/information/fuel-consumption-testing-scheme.asp
The three primary flaws:
1. Allowing plug-in hybrids to pre-charge, and then fully use up, their batteries during the test. This is not a reflection of real-world motoring.
2. Ignoring aerodynamics by running the test on rollers so the vehicle remains stationary for the whole test. This benefits large, unstreamlined vehicles like 4x4s and negates the efforts of manufacturers who try to create more streamlined cars.
3. Ignoring the mass of the vehicle. This is also a consequence of running the test on rollers, and makes the test effectively a test of the powertrain. It means that a 1,000kg supermini and a 2,300kg 4x4 will achieve the same efficiency if they share the same drivetrain. This is obviously ridiculous, as in real world driving the 4x4 will have to accelerate, brake and corner an extra 1,300kg, equivalent to carrying 17 average sized passengers!