Go ahead and get the rear end installation done, then do actual measurements using either a GPS or follow a car that you know is accurate at very specific speeds. The Jim Carter website will give you instructions. If you know the true MPH and the incorrect reading on your speedometer, you can have an adapter made that will be very precise. You can test it using the same method after the installation.
Doing the math, you will need an adapter that increases the speedo reading by 20.23% if you do nothing else but change the rear end ratio. If you also plan to change tire sizes, this will make a further change. But calibrate based on the road test, not the math.
Check to see if your tranny already as an adapter on it. Some came from the factory with them (mine did). If so, I recommend that you take it off before you do your calibration testing so that you don't end up installing an adapter to an adapter. Calibrate with your speedo cable screwed directly into your tranny.
The good news is that once you get everything else done, installing the adapter is only about a five minute job.
Matt
