I drive a 1990 300SEL with the same transmission. Your RPM at 75MPH is correct for that transmission and that rear end. It is the same as mine. The SE and the SEL with the 103 engine are geared for higher RPM. I know it seems like you should upshift to a higher gear, I felt the same way when I got my first 300SEL, but it's just the way it is.

I found this place have some 722.5 trans and data information. But the price is a little high. any more data please share..thanks