In the original post, he said it "used a 1/4 tank of gas", thats what my gas guage comment was addressing. The gauge may move a 1/4 but that doesn't mean he used 4 gallons of gas. (Of course, he didn't say how far he went on any given outing, but it's implied he isn't going very far, and that he is actually using a 1/4 tank. Just wanted to throw in another angle.)
True, indicated speed has nothing to do with MPG, but if the wrong speedo gears are in the car, the speedometer AND odometer will both be wrong. For what it's worth, most speedo's read faster than actual speed, and that is by design. The odometer would be accurate though. New cars are far more accurate, to within 2 MPH at 60, but through the 80's-mid 90's, it could have been off by up to 10%. 60's-70's were no better I would guess. Ford published a service info stating this fact.
I like the idea of the GPS, will have to do that next summer to check my speedo/odo combination.