I did new car check-in and service at a Lincoln Mercury dealer in early '70 to late '72 and the mileage was usually 3-5 miles. Jeff is correct about the gas added and it was usually enough to get the customer on the road after our road test. I don't think adjusting the speedometer was illegal back then or at least not enforced as many used car dealers would run the mileage back. I can remember when GA and TN either passed a law to deal with the issue or beefed up existing laws and started to enforce them.
TOB