That is because it is a metric speedometer.
Or, if so inclined, you can get a speedometer from an early 80's medium duty [F600/800] truck. I bought a NOS one and installed it in my F150, which now proudly displays 207, 858.5 miles.
The price was unbelievable [$49.99, including freight] and inside the box was the Ford supplied directions for setting the odometer. If you are attending the Garagemahal Ford Truck Show, you can see it in my truck.
https://www.ebay.com/itm/OEM-Instrument-Cluster-Speedometer-Gauge-E3HZ17255A-For-Various-80-86-Ford-Truck/332611131080?epid=1536784025&hash=item4d712b16c8:g:B2QAAOSwpFtaxkck