I have a 6 cylinder 1989 300SEL [W126].

Can anyone tell me at what RPM should the car be at when she is CRUISING at 60mph and at 70mph?

My tach shows around 3100-3200 RPM at 60mph and around 4000 RPM at 70mph. Does this sound right?

I am wondering if there is an overdrive gear or perhaps gear four is not kicking in.

Thanks in advance.