1st Question: What type of carb or mods do you have? I have an 80 XLH with a 1984 XL Harley/Makuni carb with a #56 main jet. I am gettin about 40 MPG on the back roads doing about 45 MPH but only get around 30-33 MPG on the highway doing 65-70 MPH. I did discover that adding 1/2 oz. of Marvel Mystery Oil at every fill-up improved my milage and overall power range.......