91 Chevy G20 Cluster question

hi i got a 91 chevy g20 and i have a couple of questions about my cluster. First off i my speedometer is right on speed till around 45 then it starts to go off by 10 miles so if im going 75 im really only going around 65 maybe less i checked the tire size and its the right tire size, next my heat guage dont work does anyone know how i can fix it or how to fix it.