[Flyinghippo]

After I sell my 16' trailer I might buy two 4x8 sheets of clear stuff like plexiglass from Menards. I forget the name of it but it was not plexiglass. I have sitting around in my storage a bunch of thin "L" shaped metal with the holes already pressed into them. My plan is to build a metal frame under it with the "L" metal and then place the 1st sheet from near the top/back of the cab toward the tailgate. Then the other piece would be cut to fit the sides like slices of pie. I'm considering having the main piece from cab top & back to the tailgate to be curved and not just go straight from point to point. My question now is how much mpg gain could I see with just this mod?
Driving to and from my job is 4.1 miles each way through town on 30 mph roads. But I deliver pizza. I probably average 60 miles a night with about half of my miles on roads that are 50, 55 or 60 mph speed limit. The rest are all 30 mph limit. Since I am going in and out of driveways, backing up etc I absolutely don't want something on the back of my truck that would block my side or rear views. My truck has a.c. which I use part of the time in the summer. My truck is slower taking off from stop signs and stop lights if I have the a.c. on. What I sometimes do is turn off the a.c. when taking off from stop lights and stop signs. I have seen a gain of 1 mpg doing this. I don't take many trips and I only get out of town on the highway 3-4 times a month.