Get a decent GPS system to check your speed. Get the percentage error of the OEM baseline at highway speeds before you start. The whole calculation is a simple ratio, so you can just increase or decrease the overall diameter by any percentage to change your speed reading up or down by that percentage. Mine read a few percent high on the OEM tires and wheels.
You can probably change the tires and rims and choose the size carefully to make it read closer to true that it did from the factory if you care much about it.
One of the better calculators:
Tire size calculator
There was another I liked, but I can't seem to find it right now.