So what is it?
Quite a bit, surprisingly. Mercedes has taken each car and fiddled with it to the extent where neither the A180 CDI ECO nor B180 CDI ECO will cost you a penny in annual car tax.
Ride height is lowered by 10 mm and 15 mm front and rear on the A-Class, 20 and 15 mm on the B-Class. There's a more aerodynamic grille, longer gear ratios for the manual transmission and even a lightweight fuel tank. The A180 has a drag coefficient of just 0.26 - Prius-rivaling without the divisive Prius shape.
What powers them?
Replacing the A180 CDI SE and B180 CDI SE, each gets a 1.5-litre turbocharged diesel with 109 horsepower. That's not a lot and you're looking at a 0-60 sprint of around 11 seconds, but economy is the main focus here - 78.5 mpg combined for the A-Class and 74.3 mpg for the B.
Emissions are low too - the A180 drops to 92 g/km from 98 grams, while the B-Class is now on 98 grams itself.
Anything else I should know?
They'll benefit you as a company user too - the A-Class's Benefit In Kind company car tax rating is just 13 percent now, with qualification for 100 percent capital allowance until 2015.
How much do they cost?
21,965 on the road for the A180 CDI ECO SE, and 22,950 for the B180 CDI ECO SE. That's an increase of 520 each over the respective SE models.
Do I have alternatives?
Plenty. Starting with the A-Class, there are BMW 1-Series and Audi A3s which offer similar frugality. We'd opt for the criminally-overlooked Volvo V40 D2 though - not only does it do up to 83.1 mpg, but it looks damn fine too.
For the B-Class you might struggle, as neither Audi nor BMW does a mini-MPV. You'd spend more on their small crossovers - the Audi Q3 and BMW X1, or you could save some dosh and get the excellent Ford C-Max in 1.6 TDCi trim. The money you'd save would pay for the 30 a year car tax...
In a line?
Mercedes' babies get the ECO wand waved over them.