On some of cars where the speedomter isn't working, or if a rear axle ratio change has made your indication off, here is a suggestion.  I think I have 3 or 4 generations of GPS units.  The oldest on has a bad battery and is out of date.  The cost for a new battery and updating are more than a new unit.  I use my oldest one for a speedometer, which is a good temporary solution.  It works off of the vehicle 12 V, even though it's battery is dead.

Paul J. - '57 E Code