[Wreford-Brown]

I believe that 9 knots is the most fuel efficient, and there's a way to set your 'ahead standard' to any value you like.
Details here from Average Joe - one of the original pioneers who is unfortunately no longer with us:

"Here's a small speed-change I made to the VIIC class submarine .cfg file (NSS_Uboat7c.cfg).

Default values:
[EngineProperties]
AllStop=0.00
AheadSlow=0.40
AheadOneThird=0.57
AheadStandard=0.80 <---CHANGED to: AheadStandard=0.47
AheadFull=0.94
AheadFlank=1.00
BackSlow=-0.26
BackStandard=-0.40
BackFull=-0.53
BackEmergency=-0.66

This helps me in 2 ways:
1.) Submerged, and 'accidently' creating a new plot-course, especially when extremely close to a pack of DD's, or convoy merchants.
Since SH3 defaults to 'Ahead-Standard' when plotting a new course, sub-noise (props, electric motors, etc.) increases greatly! If this happens I usually get detected; mostly during critical points when trying to slip past escorts, and into a convoy of merchants.
2.) Setting for best fuel-economy cruise speed with the VIIC/42.
The VIIC/42's best 'econ-cruise' speed seems to be right at 9-knots. The change above sets 'Ahead-Standard' setting to 9-knots."

You can, of course, adjust the 0.47 value to change it to your preferred cruising speed. I used trial and error but I think this value is a % of max speed.