S-class with the 3.0L six should have a 3.46:1 axle. You can also cross check this with the following forumula.

Axle ratio = revs at 60/tire revs per mile

It should be within a few percent.

You can look up the tire's revs per mile by checking the "specs" for any 205/65R-15 (the OE tire size) tire on The Tire Rack site. The speed/rev relationship holds regardless of installed tire revs/mile. If the installed tire revs/mi are different than OE it will just add error to the speedo.

Duke