I doubt the Monza car would be using as much throttle as the Monaco car. Therefore it would use less fuel even if it took 15 seconds longer to complete the lap at Monte Carlo.
I think it all depends on how much RPM is being generated.
"And Mansell was lucky not to be taken off by that RIDICULOUS bit of driving by Alliot!!" - James Hunt "AAAAAND into the pitlane..... OHHHHHHH!!!!!!!!!" - Murray Walker