The thread below relates to a totally different vehicle, but I put up some graphs related to your engine (posts 47 and 48). Perhaps they will be helpful. For instance, if you want to minimize fuel consumption at 60 mph, you would choose a diff to ensure that the engine was turning at 2400 rpm. That is the minimum Brake Specific Fuel Consumption for the 617A. Most of the original gearing choices on these cars were to maximize top speed. So if that is not important to you, perhaps a taller top gear ratio would be preferable.

http://www.peachparts.com/shopforum/diesel-discussion/321636-diff-ratio-gearing-analysis-4.html

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).