ah sorry.. not going to be easy then.
not sure if it work, but how about driving on a very long straight on a provate road somewhere... getting a sat nav readout at 10mph exactly, and place the needle there... obviously not going to be easy to do... but is one way i suppose
or just keep trying the needle in different positions, setting it higher and higher until you can get it to replicate the read outs you get on sat nav?