I don't know where fiat500abarth's "120mpg" figure comes from for the future 900 twin. They are saying mileage will be 15% better than the 1.2l which gets something like 55mpg (US) in combined driving.
That said, the EPA figures for diesels here in the US are pretty accurate.
But I agree, Rob, one of the reasons I use fuelly is for real-world numbers from real people driving production cars. The EPA has started falsely inflating American cars (see Ford Fusion Hybrid or Chevy Equinox EPA ratings), so any shred of accuracy they had before they merged with the American auto industry is suspect. Ever since they started messing with the ratings in 2007 they've been off, and now that they're exercising a major conflict of interest they are even further from the truth. I get my EPA ratings from Fuelly