Your car has a 1:1 fourth gear with a 3.27:1 axle ratio, and the OEM 185/65VR-15 tires are speced at 849 revs/mile.

849 x 3.27 = 2776

These are your revs at 60. At 75 the revs are:

75/60 (2776) = 3470.

My five-speed has the same axle ratio and a 0.80:1 fifth gear so revs at 75 drop to: 0.8 x 3470 = 2776, same as your revs at 60.

The fifth gear yields more relaxed cruising, but due to the torque shy nature of the engine, a down shift to fourth is required for long mountain grades, and from highway speed I downshift to third to pass since the engine doesn't really pull strongly until 5000 revs. In the last ten years, Mercedes and other OEMs have greatly improved torque bandwidth, so modern engines don't feel so soggy on the bottom end. Because of the soggy bottom on the old inline sixes and only four forward speeds on the automatic, D-B geared them rather short for good performance.

The taller top gear on the five-speed manual also improves fuel ecomony. From memory I recall the EPA numbers with the automatic are 18/25 and 18/27 with a five-speed. The quest for better mileage is why many automatics now have five speeds, and six speeds are in work.

Duke