Different cars have different RPM ranges where they are "happy". My Vette for example has spent most of its life below 1000 RPM in 6th gear. My MT HCH never goes under 1000 RPM because it lack the torque to move the car so it must make up for it with RPMs.

Check out the Vette at:

http://www.greenhybrid.com/compare/mileage/car/327.html

Your car is much more powerful than my HCH but not nearly as powerful as the Vette so I imagine it should be happy somewhere in between. I highly doubt your car gets better mileage at 60 than it does at 40-45. If you count your time and trouble it may make more sense to drive faster but almost for sure you will save gas by driving slower. The biggest factor on mileage at high speeds is aerodynamic drag. Aerodynamic drag goes up with the square of speed. There is over twice the aerodynamic drag at 60 as there is at 40 hence the reason for my doubt.

Perhaps you could be more careful with the throttle at the lower speed and only use enough gas to keep the car moving. Stomping on the throttle in high gear at low speed is going to eat gas and not really make the car move any faster.

good luck