YOU CAN NOT figure gas mileage by "a quarter tank" or " a half tank." You MUST use an accurate speedo (OR A GPS) and you must average it out over several tanks.
I've bought two GPS units off CL for less than 40 bucks apiece. The cheap one I have now gives me (on one screen, at the very least)............speed, "max speed," and miles traveled, like a "trip" odometer. This is exactly what you need. Just record the trip reading at a fill up and that is your miles.