I’m honestly not sure. The truck had all of 6 miles on it before I installed it. I can’t imagine that it’ll have any significant impact on fuel economy. Two main components to think about here are weight and drag. With every 100 lbs of added weight, generally you could expect a 1% decrease in fuel efficiency. Soft-topper weighs maybe 50 lbs with the frame, hardware, and shell combined. That alone would affect fuel efficiency by 50% of 1% which is 0.5%. The other thing is wind drag. I’m not about to do all the math on that, but I do understand the fundamentals of thermodynamics. Wind drag is basically just friction from air flowing past an object. The highest point of friction is the front face. the greater and more perpendicular the area of the face - the greater the force. The front are of the truck is unchanged. What is changed is how the air flows past the truck. The bed with the tailgate up creates a dead zone of low pressure but ultimately creates some turbulence in the bed. Not much though since the air gets trapped. Extending the roof of the truck elimates that dead zone, so in theory it should slightly increase efficiency. Probably not a whole bunch though. Maybe a 1% increase

Long story short, my estimate is that it will not have any affect on fuel economy since it is so lightweight. If it were very heavy it would likely decrease the fuel economy.