I think the VRod is engine is as close as it gets, but at 1130ccs I believe. The fuel mileage reports for VRods are not that good, at least compared to the 1200 Sportster.I mean 50s to 60s is a pretty broad range.
Have they used this engine in any other bikes before? If they haven't it makes it pretty hard to make a real guess about the fuel economy. I think we will need more info on the HD Street before we can give a more educated guess.