I don't see anything wrong with a 2.65/5 speed behind an OM603. The gearing with the 4 speed auto is way too short in my opinion especially at highway speed.

OM603 auto
mph / RPM 1000 2000 3000 4000 4700
1st Gear 7.6 15.2 22.9 30.5 35.8
2nd Gear 11.6 23.3 34.9 46.5 54.7
3rd Gear 19.5 38.9 58.4 77.9 91.5
4th Gear 28 56.1 84.1 112 131


OM603 + 5spd
mph / RPM 1000 2000 3000 4000 4700
1st Gear 7.3 14.5 21.8 29.1 34.1
2nd Gear 12.9 25.7 38.6 51.5 60.5
3rd Gear 20.3 40.6 61 81.3 95.5
4th Gear 28 56.1 84 112 131
5th Gear 35.1 70.1 105 140 164

On the subject of W201 diffs, the 2.3 gasser and 2.2 diesel got the 168mm diff. The 2.6 M103 and 16v M102 got 185mm diff.

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CENSORED due to not family friendly words

The 2.5 turbo is the 602.961, not the 603. I think plenty of people keep the 2.65 differential, but most agree that it is too tall (i think that's right) and believe that the 3.2x is ideal. Thanks for the info though, in post #25 I'm trying to develop a running list of ratios for the 185mm gears.

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Spencer

87 2.5L 190D Turbo 5-speed - 102k (Daily Driver)
82 300D Turbo - 145k - Light Ivory (near mint)

Gone: 85 300D Federal - mileage unknown