Speed Sensor problems of the 2003 Subaru Legacy

The speedometer/odometer is not accurate. I have checked it with radar, gps and multiple measured miles with a stop watch and have consistently calculated an 8-10% inaccuracy. For example when the car is traveling at what the speedometer reads as 60 mph, the radar,gps and the measured mile all indicate a speed of 54-55 mph. The dealer has never tested my vehicle other than driving it next to another vehicle and comparing the speeds. This is obviously not accurate . I have voiced my complaint numerous time to the dealer and to the factory. The factory authorized the speed sensors to be changed. No cure. The problem remains and worsens as the tires wear and their diameter becomes smaller. If the speedometer is inaccurate the odometer could also be wrong and this would mean that my mileage would not be as great as is indicated and my warranty would expire that much sooner. Also my gas mileage would be less and my maintenance intervals would be sooner and I never really know what my true speed is. The factory indicated to me that they would only accept dynonmometer test and that I would have to do the test at my expense. They do not have such a machine and would not help me any further. I hope that you will be able to help with this problem. I have talked to other 2003 Subaru Legacy outback owners and most of them indicated the same problem after they were made aware of this. I was told by a mechanic that 2003 was the first year the speedometers/odometers were electronic. I don't know if this is true or not, but I'm very sure that myself and other Subaru outback owners have inaccurate speedometers. Hope you will investigate this.