Rear end gear ratio's
 

I have a 1980 300sd and I need to know if there is a rear diff I can put in for better highway speeds ?

you have a 3.07 diff which should be good enough for high speeds and I wouldn't mess with it. mercedes put that axle in this car for a reason.

you can put a 2.88 from 1985 only w126/w123 300 turbos

there are even lower ratios but the yoke size is different.

You can cruise at 80+ with that gear if it is running right. How fast you wanna go?