WHICH Green Machine?

The May issue of the UK consumer magazine WHICH? has an article on what it calls "eco-friendly cars".
The Prius comes out as "44% greener"
The Lexus RX400h 3.3 SE-L as "27% greener"
The Honda Civic 1.4 IMA Hybrid ES as "19% greener"
The Citroen C3 Stop & Start as "6% greener".

Greener than what? I'm not sure - perhaps I should read it all again.

On the Prius the final verdict is that it is "quiet and relaxing to drive, if a little under-powered on faster roads. It's a genuinely 'green' family car, but you'll need to cover many miles before low running costs offset the high list price."
The prices for the three models available in the UK are quoted as £17,770-£20,270 ($32,826-$37,444)