Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire and rear end gear ratio you want. This will allow you to get the accuracy with within 2%-3%. The drawback is that it isn't cheap.
There is one in the Orlando area where I live. The the sign is a giant speedometer with a moving indicator needle. It's been there so long that it is a landmark, part of local history.
Trucks need their speedos calibrated quite often. The speedo shops carry all the little gears for the reduction box. When I had mine calibrated the guy tested mine on the dyno and picked gears from many bins, buttoned it up and it was DNO.