Odometer is direct drive thus dependent on correct gear in the trans for its accuracy. There is no direct drive in the speedometer so that needle could report any mile per hour while you travel a million miles. It is actually 2 instruments in one assembly that were once calibrated to work together. One reporting the cables revolutions only, the other reporting the cables rpm.
So... with the correct gear in the trans, odometer will be correct due to its direct drive. Then the speedometer can be calibrated to correct MPH should it be wrong.
Trivia, back before radar , police/highway patrol vehicles had routine speedometer check/recalibrate. Why? The fact that they are not direct drive means they could begin correct and slowly change.
I've seen only 1 speedometer that had a dated "certified" sticker on it.