I have owned cars with speed gauges showing two concentric graduated arcs. If I remember correctly the outer arc was MPH and the inner arc was kp/h. My current car has an analog speedometer and a digital screen in the instrument cluster, if you go into the settings menu the units can be selected. Since the computer controls the gauges and not a spinning wire from the transmission, it can use the same numbers for either unit.
On my vehicles with analog clusters the odometer is stuck in miles. The only thing I have found the odometer useful for when driving is knowing how far it is to my next destination. If I look at my directions and it says to continue for x miles I can glance at my odometer and do a little mental math to figure out what the reading should be at my next way point. Assuming I can remember the number I came up with I can subtract it from the current reading to figure out how much further I have to go. I have several devices which can do this for me automatically now, some even give other useful information like which lane I should get into.