Ive got an issue with my speedometer where it is reading incorrectly. When im doing 70mph for example it says im doing 120mph and just under 200kph.

The odometer is working correctly though. I tested it by putting a route into my sat nav to get the distance in km. I then reset the odometer trip and completed the journey and the distance covered matched the distance the sat nav gave.

Any ideas what would be causing the speedometer to read incorrectly?