Hello to all from Vegas

Its been awhile that I havent visited this forum and im so amazed a lot of interesting info has poured in since my last visit.  I have a 2001 S430 and my family loves it except for the gas mileage, 22 mpg highway and 15 city. I envy people having the same model and enjoying 25 mpg and up!  How did they do it?  I got no clue.  I changed all what is needed that i know of to improve mileage but no avail, plugs, wires & coils, engine, trans and dif oil, air, trans, oil filter, even cabin filters and radiator coolant, replaced tires and wheels, aligned and balanced it, still no change except for the cabin smells pretty good. Unfortunately,  one thing that I noticed after changing all oils, trans starts to hum at 50 to 60 mph and goes away when it reached 62 and above.  Is the car telling me that something is going bad?  Thanks in advance to anyone who can help.  Have a wonderful and safe Holidays!