I have a 1972 8KCAB with 150 hp c.s prop. I have owned the plane about two months and have always had a problem with the airspeed indicator showing approximately 10 mph faster than actual ground speed in no wind conditions. GPS confirms this and stall horn confirms this. I blew the pitot line out to check for water(none) and the airspeed indicator(none). The pitot line looks good with no signs of holes. Has anyone experienced this? I have come to the conclusion it must be a bad airspeed indicator, but I cant find a replacement. Any advice would be appreciated.