I know exactly what you mean. Before the 6 speed my car would zing around 3000 rpm at highway speeds, but never seemed to care much. The noise would get to me though. My old motorcycle had overdrive which did a nice job of lowering the pitch of the exhaust even though the rpm didn't change much compared to my new bike. The new one winds 5000 on the highway and just kinda gets to me on longer trips.
Like Rat my car was actually rather peppy with the non-OD setup as it was already wound up and ready to go. And the mileage thing is really more a function of load compared to rpm. Yes, you'll have some more parasitic drag from friction and stuff at higher rpm, but ultimately if you make the same power at 50% throttle and 1500 rpm or 10% throttle and 3000 rpm you're burning roughly the same amount of fuel. Engines are more efficient at certain speeds over others though, so some might show larger variances between speeds. I know mine has pretty much been stuck at 16 mpg through it's entire life. 318 with a 4 speed and 3.07 gears? 16 mpg. Carb'd 5.7 hemi with 4 speed? 16 mpg. Fuel injected 5.7 hemi with 4 speed? 16 mpg. Fuel injected 5.7 hemi with 6 speed overdrive? 16 mpg... I know my issue is in the tune now though. I think I've squeaked another 1-2 mpg with some massaging and bet I can get close to 20 with some more time and logging.