I find long tunnels with the roof down are the perfect cure for this obsession with fuel economy 
:evil:
(Nowadays i'm happy to get 25 mpg, the S54 is a greedy bugger!)
I would say an rpm between 2000 and 2500 is ideal for economy (not wanting to labour the engine as I found that on my old 3.0 it would return better fuel economy at say 4th at 30mph than 5th as it was more comfortable at that engine speed). Next gearing and aerodynamics are pretty important in deciding how much fuel (energy) you need to hold you at your desired speed, overcoming mechanical (turning the engine,drivetrain and wheels) and air frictional forces. With the roof open the drag becomes significant as speeds increase, a slippery coupe or roadster with the roof off should return better figures in that regard but its still hard to push a car through the air at three figures!
At the end of the day the biggest factor in deciding fuel economy is the driver and type of roads, some times I get sub 20mpg (B road 'fun'), others 30mpg steady on the motorway with no large throttle openings or dropping a cog to pass.
:evil: (Nowadays i'm happy to get 25 mpg, the S54 is a greedy bugger!)
I would say an rpm between 2000 and 2500 is ideal for economy (not wanting to labour the engine as I found that on my old 3.0 it would return better fuel economy at say 4th at 30mph than 5th as it was more comfortable at that engine speed). Next gearing and aerodynamics are pretty important in deciding how much fuel (energy) you need to hold you at your desired speed, overcoming mechanical (turning the engine,drivetrain and wheels) and air frictional forces. With the roof open the drag becomes significant as speeds increase, a slippery coupe or roadster with the roof off should return better figures in that regard but its still hard to push a car through the air at three figures!
At the end of the day the biggest factor in deciding fuel economy is the driver and type of roads, some times I get sub 20mpg (B road 'fun'), others 30mpg steady on the motorway with no large throttle openings or dropping a cog to pass.