I strolled into some car showrooms last weekend and tried to get the low down on fuel efficient cars.

I got a few strange looks from the salesmen when I asked about CO2 emissions. One was puzzled about exactly why I was interested in emissions levels. Though there were eco-labels on most of the cars, I got the impression that no one had ever asked him before. "Was it to take advantage of cheaper car tax?" he asked.

"No," I told him. "It's because I want a car with low CO2 emissions." That clearly didn't make any sense to him, but to be fair, though he didn't know why exactly I was interested in a low emissions petrol car, he did catch on and get me all the data and show me a new petrol car that when it's released next year will come in at just 99 g per kilometre, the lowest emissions for a petrol car on sale in the UK.

© Frank Mentha / Greenpeace

But the one big point about buying a greener car that he just didn't mention is that lower CO2 emissions equals lower fuel costs. It's as simple as that. And in these credit crunch times I reckon it's a brilliant sales tool.

If I chose something like a petrol-driven Citroen C1 with emissions of 109 g, instead of a C3 with emissions of 143 g per kilometre, I'd save myself almost £300 (around €350) in a 12,000 mile year's worth of driving.

I looked at petrol engine cars - diesel may be cheaper but there are still problems with the particles emitted that cause other kinds of pollution, so I decided to stay away from them. And I set myself an ambitious target, more ambitious that the 2012 target being considered right now by the EU, I chose to slash that by 10 g and looked at what I could get under 120 g per kilometre. If I had gone for 130g that would cost me roughly an extra £124 per year. So these are significant savings.

And I found that there are 26 models to choose from, which sounds pretty impressive. Though when you drill down a bit there are only twelve manufacturers. And looking more closely I found that some of them are basically the same car - the Peugeot 107, the Toyoto Aygo and the Citroen C1 are triplets.

There are a couple of hybrids in there - the Prius and the Honda Civic hybrid.

I went into three car showrooms and none of the salespeople mentioned to me that I could save a minimum of £100 a year in running costs just by switching to a car with lower emissions. All of them mentioned the fact that at 120g or lower the UK annual car tax is £35 (that's £85 cheaper than cars with emissions of 121 -150) but why didn't they add on the up to £300 saving in fuel costs and tell me I could save nearly £400 (that's around €475) per year? The tax bands are a great incentive - everyone likes to pay less taxes, but if you use your car regularly it's the fuel cost where you can make some significant savings and it would be the potential £300/€350 saving in petrol that could persuade me to choose one model over another.

And while there are a lot of smarts on that list of 26 cars, there are none from Audi or BMW, only one Peugeot, one Citroen, and two Fiats - both 500s, which seem supersized when you compare then to the original iconic cinquecentos still sneak in at 119g per kilometre.