speed error
New to this so bear with me. Bought a 1930 Tudor been sitting for a couple of years. Rewired complete, changed all fluids adjusted brakes put in a 6v alternator and electronic ignition. Old speedometer did not work so got a rebuilt on. Now comes the issue. Car speed showing is 38mph but my phone gps puts speed at 50mph. Everything is about 12mph off although slightly less at real slow speeds. What is the best way to correct?
Thanks
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