We own both and I'd almost agree with that - the smart uses less fuel 60mpg is an easy average where as my A2 is about 52-55mpg, but the smart uses 1L of 0W40 fully synthetic oil every 3000 miles to counter the balance.
That's right - for a few years now, new cars have been taxed on their CO2 emissions, I think and the 1.4 petrol pumps out 144g of the stuff, which equates to £115 a year.
I don't know the figures off hand for the Diesel, but I remember after the last budget speech there was much joy and jubilation on here from the Tdi owners when they were given a reduction in tax to £35.
This applies to post March 2001 cars only I'm afraid, cars pre-dating that are rated on engine capacity only
Mike