Hey bro, try using an app on your phone that tells you the speed of your car using a GPS, that's more accurate reading than matching a car side by side.
Also there are a number of factors here, as other members pointed out, speed indicated is always higher than the actual on speedometers, that holds true for even brand new cars. Even top end sports/hyper cars have same issue.
If you're running bigger rims and tires, that can also be an issue here, so I think you should try to live with it rather than try to recalibrate the speedo which I don't think is possible.