My car gets 32-33mpg on short trips. I will say if it gives me more than 34mpg than it's a success. I have also pumped my tires to 3 more psi so I hope this combo gives me 35mpg at 65mph. I can get a lot more mpgs but I don't drive 55mph just to get more than 40mpg.
just to be clear, what you're doing here is not really a diffuser. it's a belly pan.
diffusers have a carefully designed shape meant to accelerate air as it exits the rear of the vehicle to generate downforce (reduce lift). a diffuser may even increase drag (but its downforce benefits high speed handling).
What did you do by the muffler? It looks real good.
I used duct tape and attached it to the muffler heatshield. If this belly give me good results I'll redo it, not make it look ghetto. So when somebody walks up to my car it will be completly hidden unless they look under the car.