depends on my wheel set. In winter I have my snows on 997 Turbo replica wheels that are porkers. Proably 34 #'s. Add the heavy ***ty snow tire. Weight is up there. Fuel economy drops by a solid 5 mpgs.
In summer I have the the factory Carreras and the super stick RE 11's. Probably 1/3 lighter.
I have no problem going over the 300 miles mark ona full tank.

Cruise on the highway was about 535 KM long and my milage was 36 MPG Imperial and not being light on the happy pedal. It was Shell 91 which is 100% gasoline, no Ethanol.

My MPG is directly related to my right foot! Since I still love the sound of that flat 6 winding out, I'm afraid I can actually watch the fuel gauge going down as I wind out the gears. Actually could care less at the moment the MPG, still having too much fun.