Figure your average fuel milage and leave 1/2 gallon reserve. I gradually went from filling up at 110miles to between 130-140 miles. Usually takes between 2.7 & 3.0 gallons. I don't know why, but at about the two year mark, my fuel guage started getting more accurate. Originally the guage would show empty at about 80 miles, the light would come on about 100 miles. Now the guage gets to empty about 110 and the light comes on about the same. Also fuel milage average went from around 43 to 48 at about the same time which partially explains the guage, otherwise beats me. Mine is an '07 carb. model.