Type IXD2 getting lousy mileage; ..insights?

[Greystone]

Hello people;
Traded in my type VIIC for a type IXD2 at Lorient in hopes of far off adventures.
Set off for patrol grid DT37 to log my 24 hrs there and, when done, had gone about 4300 km total.
The indicator on my fuel reserves shows about 52% diesel remaining and my navigator declares that I've got 9600 km left at ahead 1/3.
I've kept it at ahead standard to give me at least some speed for dive time to elude air attacks, and used battery power as little as possible to minimize charging time while surfaced, when I'd run it at ahead full until the batteries charged, then dropped back to ahead standard.
These things are supposed to go 2300 nautical miles.
I'd like to go to the West Indies or US East Coast, but there's no way I'd make it there and back, much less do a search.
Do I have to run consistantly at slow or ahead 1/3 to get to these destinations and still spend some time searching for targets?
Thanks!