nodak

Hello...new here and new to fords...I just bought a 1997 ford f-150 regular cab...love the truck, but have a small problem. I installed 255 - 70 R -16 tires which were an option on my model, but the speed is 6-7 MPH off at 65 MPH, which unfortunately the law enforcement community frowns on. I was told the speedo could be adjusted by computer. True? Only by ford garage if so?- Thanks for the help.