Is a Toyota Prius really greener than, say a new BMW 118d? The hybrid Toyota might offer lower CO2 emissions per kilometre, but how much more CO2 is created in manufacturing it?
New-car buyers could soon be in the know, if a government initiative in collaboration with the British Standards Institute and the Carbon Trust goes ahead.
The plan is to develop a method for measuring greenhouse gas emissions in the manufacturing of all products.
Primarily this will be to allow firms to measure the impact their products have on the environment, but it should also lead to labelling that helps consumers buy greener cars.
Inevitably it's extremely hard to account for all of the CO2 produced in manufacturing.
In 2006, US American market research company CNW produced a report that measured the amount of energy cars used throughout their lifecycle, from manufacturing to running to disposal.
Controversially, it indicated that a Jeep Wrangler emits far less CO2 than a Prius when its manufacturing and longer life expectancy are taken into account, despite far higher exhaust emissions per kilometre.
There's no indication yet when the new ratings will be introduced. A government spokesperson said consultation with manufacturers is due to begin this summer.