I'm just trying to head off the likely disappointment you will be facing after replacing this bike with another Ultra powered bike. There's no magic involved. If you can't get over 40km range with the bike you have, with another bike, even if it were dripping with quality and weighed half of what your current bike weighs, I doubt seriously you're going to see a significant increase in your range. Not without some changes in how you are riding the bike (assuming the bike is right/nothing wrong)....
At half of my weight, that bike should be able to run 50 or 60km pretty easily.....
Another idea, maybe stupid, but are you keeping the tires inflated to something near their max capacity? If they're too low, it could be like towing a boat anchor around in back of you, easily dragging your range down to what you are seeing.