Close the Gap
The front of the Cascadia Evolution is aerodynamic. The front of my trailer is not. Together they form a unit. The idea is to get them to be as aerodynamic as practical as a unit. Trailer skirts and trailer tails help. Under tray systems help. They can offer a substantial return on investment. Closing the gap between the truck and trailer does not cost a dime. The only investment is the time it takes to slide the fifth wheel.
I have always heard that 18” is the best distance between the cab extenders and the front of the trailer. I know that if I put my hand right behind my mirror as I drive my truck there is no resistance. As I move my hand back it meets more resistance from the wind. At some point my hand meets a lot of resistance. 18 inches sounds about right. So far I have not held a tape measure out the window as I drive to get it exact.
My last truck had 12 inch extenders. My new one has 20 inch 
extenders. Even though my fifth wheel is in the same position the extenders let me close that gap. It helps. I drove my old truck 126,000 miles. My best tank was 9.21. My new best is 9.45. The average tank has been about .31 MPG better. This truck has the condo sleeper. That improves the flow over the top of the trailer a little bit. It also has wide based singles that give me a rolling resistance as well as a turning weight loss. Some of that rolling resistance is lost on the new truck because I am running a better traction more rolling resistant Michelin XDN tire as opposed to the XDA that I had on the old truck.
I can't spin the truck quite as sharp as before, but it will turn sharp enough to reverse pivot the trailer axles. That is sharp enough. Moving the fifth wheel forward puts more weight on the steer axle. Almost every state allows 13,000 pounds on your steer axle. Check your axle spec. It should be on the inside of your door. If it is limited to 12,000 pounds, so are you. It is easy for you to check. It is also easy for the DOT to check. Your tires may also limit the amount that you can put on your steer axle. Many tires have a 6,175 pound rating. That limits the amount that you can have on the steer axle to 12,350 pounds. Check your tire ratings. They are on the side of your tires. DOT can also check.
So far I am pleased with the decision to slide my fifth wheel forward and close the gap. I don't notice any difference in the ride. It is a decision that you can make for yourself, unless you have a fixed fifth wheel. Every .1 mpg improvement means about a $685 fuel savings for me. My estimate is that I am saving close to $1,000 per year by closing the gap.