The odometer is gear driven all the way to the numbers. so you make it accurate.
The speedo needle is driven by a gear to a magnetic driven spring. hence why you see it bounce every once in a while or lag
This part is adjustable on most speedometers.
Thus the reason lots of guys wonder why their milage is off when they fix their gears to a speedo that is out of Wack.
on the 55-59 series truck for example you slide a bracket to adjust as it "ages" or while cleaning and reassembly it needs adjusting.
some other years have a screw etc.
i'll track down the tech tip
