Type IXD2's 23700 NM does not compute!

[GerritJ9]

H.T. Lenton's "Navies of the Second World War: German Submarines Vol.1" page 107 states:
O.F. 442 tons; 23,700/57 miles at 12/4 knots.

The German "Geschichte des deutschen Ubootbaus" by Eberhard Rössler gives the same figures for range on page 452, though does not state fuel capacity.

One thing to remember is that diesel-electric propulsion is less efficient than pure diesel propulsion- there are, first, electrical losses in the generator, then electrical losses in the electric motor; pure diesel propulsion does not have these losses. So, assuming the same propeller efficiency and same fuel load, a diesel-electric vessel will have a lower range than the diesel-only vessel.