Diesel consumption too high in type IXB

[ref]

Originally Posted by Archive1

It seems to me that the fuel consumption for the type IXB is way too high. Historically R. Hardegen (U123) went 8,918 NM with only 50 NM submerged in second patrol to the US East coast (Op Drumbeat) in 1942. I tried to duplicate this: required at least 50% time submerged, surface speed ahead 1/3rd and arrived at US coast with only enough fuel remaining for 3,000 Km (not, note, 3,000 NM) - no way to get back to Lorient. Should point out that Hardegen did not refuel at sea and ran out of fuel in the approach to Lorient - entered surfaced with E engines.
I cannot comment on the type VIIs.
Any comments on how to revise diesel consumption downward?
Running GWX1.3 by the way.

Had you have a lot of wind in the travel? it affects fuel consumption.

Ref