As birdman and mcarnage59 have said, the only things that will affect your indicated speed vs actual (provided you haven't changed the diff ratio) are the diameter of your wheels/tyres and the drive gear on the transmission end of the speedo cable.
Once we know the diff ratio, wheel/tyre size and what kind of transmission you have we can calculate the number of teeth you should have on the end of your speedo cable. It's then a simple job to yank this out and check against what you have now.