Both products will be based around the new Fortwo city car that goes on sale next month.
The first will be the left hand drive MHD, for Micro Hybrid Drive, and will arrive in showrooms early next year with a £150 price premium over the standard version.
It features stop-start technology and will be available as a coupé or cabrio in all three trim levels. Fuel economy rises from 60.1 to 65.7mpg, and there’s a reduction in CO2 emissions from 112 to 103g/km.
The second model will be Smart’s 800cc diesel, which boasts even figures with 85.6mpg and 88g/km of CO2.
That undercuts the Prius and Civic at 104 and 109g/km respectively.
The car’s penalty for being so economic is a 0-60mph time of 19.8 seconds and a top speed of 84mph.
A spokeswoman said: “We’re not expecting a large number of orders but because of the low CO2 emissions it’s worth us offering. Like all new Smarts, it will be exempt from the London congestion charge when the emissions-based tolling proposal takes effect.”
Smart said it believed the diesel would appeal to urban-based commuters with short journeys who didn’t need particularly quick acceleration. “We will be numbers one and two in the country on emissions,” the company said.