Does anyone out there know what would be the ideal rpms (for fuel economy / driveability) for a non turbo 300d?

It seems that mercedes designed these cars to highway cruise above 3000 rpms - I have a 75 300D with a m/t conversion and am thinking of putting in a 3.07 rear end but would like your imput first. Theoretically lower rps (ex 2000) seem like they would equate to better mpg but what do ya think.