From the first kilometer on my precious developed only modest thirst. With tire pressure of 30/35 PSI it consumes 2,9 - 3,2 L, that is 87-97 mpg (imperial). Top Speed is 140 km/h or 87 mph on a flat road without wind.
@ jjoe: gearing is almost the same, the G5 has 19´wheel and 17-tooth-sprocket, the C5 has 18´ and 18 tooth. The difference is within the the slip of your tire.