Help fixing my speedometer

[Jakes-66-K10]

My speedometer reads high, about 5 miles too fast when at freeway speeds but not so drastic at slower speeds. I installed a 700r4 over the summer, and if I remember correctly I installed a red speedo gear set. I'm not sure what needs to be changed, but I could sure use some help. Is there a way to know just what gears I need, instead of ordering them one at a time and checking accuracy of the guage with a GPS? My tires are 265/75/16 with a 3.73 rear end... if anyone knows a formula for this.

I was kind of excited thinking the good gas milage I was getting, but then it hit me the other day that my odometer is also reading too high...