Bob:

There will be a change in the speedo with the final gear ratio change. Mathmatically it should be the percentage of change vs. the original ratio. For example, since our car is currently geared at 3.27:1 and you install a new 3.69:1 differential the percentage of change is 13% which means your speedo would read 13% faster. The same would hold true if you put smaller wheels on the car.

The speedo is calibrated for the current gearing and wheel size and can be recalibrated very easily by a good MB or DC tech / mech.

PS: I have found a 3.69:1 MB pumpkin that will fit the Crossfire and am waiting for delivery.