Verified answer

No. By adding weight it'll decrease your fuel economy. Driving with your tailgate down doesn't help your mileage either. With the tailgate up, it creates a "bubble" of air in the bed causing the moving air to glide right over the top and not hit the tailgate. Those cargo nets that they sell to replace the tailgate and "help your fuel economy" is nothing but a scam. Best bet for better fuel economy is make sure your tires are properly inflated and keep your engine tuned up. Using a synthetic engine oil will help too.

It depends on the shape and style of camper top - some might streamline the vehicle more, some might hurt the aerodynamics. No easy answer. Search Google for the model of top and truck to see if anyone has tried that particular combination before.

You won't see any measurable difference in town, only on the highway.

Edit - To the answer that says drop the tailgate for better fuel efficiency - wrong. Dropping the gate breaks up the vortex that forms in the bed, disrupting the airflow over the truck and increasing drag. Reference below..

It depends on the shell, the truck and the driver.

My truck gets better mileage with the shell than without it, but mine is a smoothie shell that continues the line of the roof, nothing poking up above the roofline. It's also not too heavy - about 200lbs. I can remove it by myself.

You can do a lot to make your truck get better mileage. Start first by slowing down. There's about an extra 15-20% of fuel mileage to be had by slowing down from 65 to 55mph. Also, be sure your tires are pumped up - the improvement is crazy huge. I run mine at the sidewall maximum, 44psi. I get over 35mpg in my truck, and that's not bad, especially considering it's 20 years old.

Go to ecomodder.com to learn about things you can change on your ride to make it go farther on a tankful.

No

It adds weight.

I can see how if it goes straight across it could help the arrow dynamics but the extra weight will just make it worse.

Although if you put the tail down it helps a tad

No, it adds extra weight and hardly and aerodynamic efficiency.

no it just slows ya down,,but you get a dry place to sleep lying down and tools stay cleaner too