35 vs 40 mpg = 12.5% lower.
Coincidentally... (?) my own unscientific observation on the same question was pretty darned close. In the Firefly: 53.4 vs 60.3 mpg = 11.4% lower.
Coasting experiment: engine off VS. engine idling - MetroMPG.com
EDIT: from this we can see why automakers really want the EPA to devise a way to take into effect engine start/stop systems (on non-hybrids) in the city tests.