AVG.RPM's

If I recall correctly RPM @ idle is 650 +/- 100 . But what about say going 65-70 mph? my 83 300D is spinning at about 3k. Is this normal at this speed. It drives fine but just SEEMS like the tach reading should be less. Maybe around 2000 instead.

As far as I know the rear end is stock, does any one know the gear ratio's?
How much power would I lose and how much would the RPM drop with a smaller rear end?
I am just curious, no biggie. as always. thanks. zachariah