I'm not surprised you lost time with the downshift. From just the ~2,000 mi. I've driven mine it's obvious the best acceleration is up to about 3,500 RPM. It'll flat move out if I tip into it at 55 or above, but D>S and flooring it just belches a black cloud and makes some noise.

Incidentally - are we talking time from when the needle passes 40 on its way up, or from a steady, sustained 40 on up?

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'82 300CD
"Pearl", the very first turbo diesel 123 coupe
Totaled 11/23/18, rebuild in progress.
'85 300TD, "Artemis".
'78 300D euro, "Ol' Red", mostly retired.
'85 300D, "Gandalf".