I think you have to look at the shift points on the dial (the little hash marks that indicate the top of each gear range). If they're identical, the speedos will be usable. If not, the speedo won't be accurate at all.
The speedometers are designed for different rear-end ratios.
The other difference is in the pick-up for the cruise control--they're different on different models.
In general, the different speedos are not interchangeable, but you might luck out.