Quote:
Originally Posted by davesrave
I check it both ways and find the car calculated MPG is very accurate. That is, assuming that the car calculates correct mileage, which is sketchy, because GPS MPH readings are always lower by 1 or 2 MPH than what the digital speedometer says it is. So, even if your using pencil and paper, you're still relying on the mileage that the car calculates which may or may not be accurate.
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The speedometer wouldn't have any correlation to the distance traveled (separate measurements) so it shouldn't impact the calculated MPG.
Also, speedometers typically read high by design. The primary goal is to never read LESS than the actual speed of the car. Most are high by one or two miles per hour but it could be as much as 10%.
Road and Track
carried a good article on the subject some time ago.