no calibration possible or needed [other than correcting the needle position] if things are as designed - the original engineers calibrated the gearing, magnetic strength, return spring and number template to be 'reasonably' accurate - if the reading is very far off, something needs replacing .... "calibration" for old trucks would involve measured miles and stop watches and making note of any error, most likely constant over the full range, same as with any of the gauges, none of them are of scientific accuracy
Bill
