I poked along at 60 to 65 mph in my '83 300SD.
I poke along @ 65 to 70 mph in my '99 E300TD. (Still gets no better than 30 mpg)
I poke along at 60 to 65 mph in my 7.3.

I'm not a fast driver, and I hate to beat-up my equipment running it hard. -Even though it can take it.