02 November 2009 by Parkers Team

  • We shed light on secretive CO2 emissions tests
  • Techniques used to achieve the best results
  • How bad results can be changed by manufacturers

Anyone choosing a new car could be forgiven for thinking official tests carried out to determine a car's CO2 output are nothing other than impartial and accurate. This couldn't be farther from the truth.

It's a secretive process that neither the Government, nor car manufacturers want buyers to know much about.

The figures are made clear when you buy - they're in the brochures, on the V5 and on websites, but how they are arrived at is an opaque process, that's one of the motor industry's best-kept secrets.

So much rides on the outcome of these figures that manufacturers leave nothing to chance.

It's not unreasonable to expect these tests to be carried out in dedicated Government labs, by Government employees.

This isn't actually the case. Manufacturers use their own facilities with their own equipment and their own rolling roads - in essence carrying out their own testing. 

The fuel consumption  and CO2 tests are done at the same time with a driver on a rolling road following a set of instructions (going through gears, up to certain speeds, idling etc).

The results from this car are then used as a base for the rest of the range.

These tests are not necessarily even conducted in the same country as the car is being sold and manufacturers can re-submit the cars to subsequent tests if they don't like the results.

That's way so many derivatives slip under important thresholds - like 160 g/km - by just one or two grammes.

So why do CO2 test results matter so much? For private car buyers it dictates how much road tax you pay and 1 g/km can be the difference between a £35 bill and a £120 one.

To the company car driver it's an even bigger deal, with small differences making an even larger impact on your tax bill.

Few people realise that CO2 emissions can vary from derivative-to-derivative.

That means an auto will usually have a different output to a manual model and even small things, like choosing optional larger alloy wheels, can make a big enough difference to change the emissions figure and possible its company car tax band.

That's why it pays to check to see what the exact CO2 figure is on the specific car is that you're buying - they can vary wildly.

If you're looking for the lowest CO2 figures, opt for a diesel. As these sorts of engines rev a lot lower, they do less 'work' and therefore emit less carbon dioxide.

The downside is that diesels can often be a lot more expensive than petrol engine models to buy.