[Christ]

You can usually pull the speedo out without removing the dash entirely. There is normally a bezel that you have to remove in front of/around the cluster.

The odometer is only ever off as a result of tire sizing, as it's gear driven from the speedo cable (or electronically driven from a VSS) the speedo is also driven by the cable, but it's driven by transfer of energy from the cable spinning a "fork" around a weight, and the weight is what's connected to the needle. I'm not really sure how to refer to the force at work there, but it's none-the-less pretty interesting when you see how it works.

What actually happens is that the speedo cable's "fork" section spins furiously around a weight that is round, but they never make contact. The "fork" is basically whipping the air, and that causes the weight to move and hold against a very light spring. The fork produces just enough energy via tight tolerances and air friction to keep the weight in a specific area per MPH, which is displayed on the gauge. This is why mechanical speedo's default to 0 (or further).

For VSS and electronic speedos, I believe it's just a signal input to an actuator which is calibrated as "pulse width input = length or distance output = speed display"

IOW - the VSS signal tells the actuator how far to move, and the actuator is calibrated to move a certain distance to display each MPH.