In my 1990 accord lx it seems that there is somethign wrong with my speedometer, it seems to read fine when im going under 50 MPH, but once I go over 50 it jumps up and says I'm going about 10 MPH over what I am really going, I know this because once I followed my mom home and called her cell to see how fast she was going and she was going 70 and my speedometer said I was going 80. I'm not sure if it is accurate under 50 MPH but it seems to be, any suggestions?

Thanks for the help!