Have a buddy drive next to you on the highway. Tell him to go exactly 55. Or whatever your speed limit is, and match his speed. Then, note the rpms and the speed your speedo is clocking, and know that whatever speed is being shown, is ACTUALLY 55.
You just have to hope your friend isn't a dick and actually does 65 when you tell him to do 55. Lol.
1986 GT with the typical poor boy mods. 3.73s, Hurst short throw, King Cobra Clutch with aluminum quadrant, lowered two inches on Eibach springs, Hopefully adding some heads and a decent intake soon!