Keep in mind that the speedometer may be accurate, or not, but that does not mean the odometer is likewise.
the speedometer is driven by a spinning magnet pulling on the needle movement, excess friction in the needle bearing can cause an inaccurate speedo reading.
the odometer however is direct gear driven by the cable, and thus does not necessarily behave as the speedometer as for accuracy the speedo can be way off do to needle friction, yet this will have no effect on the odometer.
to test your odometer, you need a known marked length run, preferably several miles to assure precision. likewise a gps can be used to check the odometer.
the speedometer may be wrong and odometer correct, or the speedometer maybe correct and odometer wrong, or they can both be wrong, or both correct.
the fact that the speedometer calibration has been found by some to be not proportionality off shows how the speedo can be wrong in a way that an odometer cant be as the odometer is gear driven, so it MUST read proportional to the drive cable turns, where as that is NOT the case for the speedometer.
So calibrate the odometer for gas mileage determination, do NOT assume that a correct reading speedometer means the odometer is correct, and visa versa.