Errors add up and GPS only measures distances assuming no elevation changes (think trigonometry, Pythagorean theorem, hypotenuse, etc) as Bravoexo said. Meanwhile your odometer and tires measure pure road distances. E.g. a mountain road distance on a map (or as a satellite sees it) seems only 2 kilometers, but in reality can be as long as 4 kilometers depending on the gradient and elevation change.



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