The way I would do it is as follows......
Connect the speedo up to a 100 watt light bulb in place of the motor.
Connect a voltmeter across the bulb.
Now, add another pot by connecting it in series with the one supplied.
Turn both pots to max,then adjust the second pot until the meter reads 170-180v dc....
Switch off and remove your second pot,measure the resistance, that is the value needed....should it be an odd value then go up to the next preferred value available.....
Pm sent.....