Scooterotica

I've got a kph speedo, but the front wheel, fork & hub, & therefore speedo worm & cable are PX. I'm assuming the reading on the speedo gauge is governed by how fast the worm & cable rotate & their position relative to the outer circumference of the tyre, so my question is what will my speedo be reading - mph, kph or neither?

i'm asking 'cos we were riding today & my mate behind on a mph speedo was reading 60mph while I was reading 60kph on mine which is 40mph