Wow that's great efficiency! You must be running a super-atomizing, dfi, diesel hybrid of sorts
I've too been using a NAVMAN and have had good results. I think they went through a period trouble prior to Navico buying them. My original unit I got from a dealer in a box that had clearly been opened and it never worked. New Brunswick Technologies replaced no questions asked.
The unit should remain as vertical as possible. Take it off and blow through it. It should spin freely and whistle with little effort. If not indeed that's your problem.
Also make sure its reading GPH. If in fact its set to MPG then it's probably ok. Ask me how I know...
