I would say from my experience with this 95 F250 When I bought it only 9 mpg with 4.10. When I plug that exhaust leak mpg went 10.50-11 mpg. But for some reason it keep pop exhaust leak where I patch. I couldn't justify drive F250 daily due 9 mpg so I upgrade to 3.55 now I get 13 but I was wonder if I get 15 mpg. I decided to replaced exhaust y pipe where it have hole. It turn to be Half of pipe are broke it leak tons. I haven't test with new y pipe because exhaust stud broke. I let you know in couple weeks when I get mpg. I talk to several people they don't believe exhaust leak cause terrible mpg. Can anyone prove it will cause terrible mpg when there are leak exhaust.