In consumerreports.org - "The Smart will likely be powered by a 1.0-liter turbocharged 3-cylinder engine producing 84 horsepower."
Boy, I'd love it with the Turbo and I don't think the mileage would suffer that much and to me Super unleaded would be more justified.
I know this is wishful thinking on my part, but I'd be willing to spring another $500 - if they did Turbo AND 4 year 50,000 mile warranty.