Once upon a time, there used to be more than several companies that offered mechanical speedo ratio adapters For VW and Porsche. (sorry, Ed - They might have been before your time 😉 . These were made for the rally set to accommodate their different tire sizes depending on rally route, competition or weather. They’re probably still out there, but the demand has evaporated in the current GPS-based world.
The “old way” we determined what we needed was to run the car on a “measured mile” either at speed (to see the distance in 60 seconds) or as a measured distance against the odometer and then tell the adapter supplier that you needed an “X” ratio adapter offset to get you to where you want to be.
I have used adapters that connect directly to the speedo OR little adapters that connected to the speedo cable and then had a short-ish cable from the adapter to the speedo gauge. THAT always seemed more logical than an adapter attached at the wheel (or transmission, if you had a different Euro car, like an Alpha).
So, if you have enough discrepancy Between your speedo and your tire circumference to worry you, then go for the adapter, but find one that mounts at the speedo gauge because it’s definitely un-cool to mount it on the wheel end of the cable.