Originally Posted by GoDeep

If you have GWX 3 installed, you should also have the manual that came with it. There is a table there that lists the U-boat ranges from "vanilla" SH3 and SH3 with GWX. It's on page 72 of the manual.

According to the table, the type VII's had a range of 12.000 miles in stock SH3, but 15.000 miles in GWX. (For why that is, see the manual, they explain it very well). The various type IX's had varying ranges, ranging from 16.000 miles for the IXB, to a staggering 44.000 miles for the IXD2!
(20.000 and 55.000 respectively in GWX)

The slower you go, the further you can travel (less rpms for the diesels). The type VII traditionally cruised at about 7 knots, which seemed to be the most fuel efficient speed. The faster you go, the more fuel you burn, of course.