I believe this would be about right for a 300D. A 240D is geared even lower and is doing about 3,500 rpm (calculated) at 70MPH.
I have the gear ratios in a book at home, I will try to dig it out and do the calculation. This, of course, is if I manage to get my taxes finished with time left over.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in