No need to worry about mileage till you get the odometer fixed. With out a accurate means of checking the miles driven, you are just guessing at mileage. Even with a good odometer, it still takes, at least for me, three or four tanks of fuel to get a accurate idea of fuel mileage. Weather from day to day, route changes, or even fuel stations can all account for different miles per gallon. You need to use the same pump, facing the same direction, let it shut off on its own, and fill to the same level each time you add fuel. This is the only way you will know for sure.
Get that truck out on the interstate that has mileage markers, you need to drive minimum of 10 miles, fifty would be better. Note the start and finish miles on your odometer and figure out how far off it is. At this point, don't worry about the speedometer, it should follow the odometer when correcting.
Since the odometer is gear driven, speed is not a factor, just drive straight and steady. Let use know how it does, we can figure out which gears you would need for the out-put to the speedometer head.
Joe
