Looking at buying a FZS600 again.

[BBROWN1664]

There is an electronic module you can get that plugs between the sensor and the speedo to convert the pulses received from kmh to mph. It effectively makes the needle point to the right number so 30 is 30. You should have a little sticker on the glass of the speedo covering where it says kmh with mph to make it all legal but its not necessary if you know.
When this box is fitted, the odometer will start to increase in miles rather than kilometres too.