Hello my friends

So I drive an old 407D from 1978, with the OM616 engine. It has no tachometer and is pretty loud, so I was wondering how much the engine actually turns when cruising. Normal cruising speed would be 75-80 kmph for this vehicle. I tried to use an online calculator to determine how much RPMs its turning at 80 kmph, and it said 3700 which seems pretty high! Is my calculation correct??

The tires are 6.50 R16, rear ratio 5.857 and translation 1:1 in 4th gear.

I am worried that the engine would take damage from such high RPMs for a prolonged time