The gear ratio is stamped into the rear of the differential housing. It is viewed from the rear looking forward and is not on the rear cover. It is on the unit itself. From the rear the housing can only be seen at on corner at the bottom. The rest is the rear cover, This small corner has the numbers.
In a 3.92 ratio the driveshaft goes 3.92 times around for one turn of the wheels. High numerical numbers 4 versus 3 say, give quick acceleration but poor top end. The engine speed can be calculated by finding the circumferance of the wheel (pi)x(d). For a 25 in tire that comes to 78.5 inches.
A car traveling 60mph is going one mile per minute or 63360 inches per minute. Divide the distance by the circumferance (63360/78.5) and the number of tire rotations is 807. Multiply by the rear end ration (since all MB trannies of that era have one to one final drive) and you get 3163 rpm (revolutions per minute). If on the other hand you had a 3.46 ratio the engine would be turning 2792 rpm.
The most valuable use of change of ratio I have seen in MBs is to take the rear end out of a 280SE 4.5 and use it in a 280SL. The SL has always had poor highway caracteristics (loud, ticky motor) which are helped immensely by the lower rpms.
Bosch Master, ASE Master, L1
33 years MB technician