At the start of my very first patrol, while entering the Kiel canal (fair weather) I interrogated
my nav officer for max range after my sub attained certain speeds. The following are the averages I got:
1 kts : 1157 km
2 kts : 1960 km
3 kts : 3017 km
4 kts : 4133 km
5 kts : 5133 km
6 kts : 5919 km
7 kts : 4565 km
I didn't try speeds higher than that because I didn't want to beach my sub in the canal. Also, it was clear 6 knots was the best speed for range. Any other speed only makes you thirsty (it's a really odd curve imho). Since the 'ahead one 3rd' is about 7.4 kts (according to "NSS_Uboat2A.cfg" in the Submarine/NSS_type folder: 0.57*max speed (13)) you'll see the reduced range you got makes sense.