speedo problem

[mustanger]

hi,all i have a 1972 mach 1 with a 351 cleveland which was suppose to have a 2v carburater .It has a holley intake and holley 4v carb on it now.I bought it 2 months ago.It had headers and the exhaust rusted out.I put on new hedgeman headers and flowmasters on it.The brakes and master cylinder was shot.I replaced them too.I had numerous light problems and fixed it too.The only problem I have now is that the car speedo is way off.Does anyone know How i can fix it.The door tag says I'm suppose to have and fmx trans with a 2.75 rear in the car.Does anyone know what size gear should be on the speedo .thanks