Due to numerous factors, it is rather difficult to accurately calculate the difference between actual and indicated speed and miles when using tires that are different than stock. What I do is calibrate my offset every year when I go to Moab. I write down my indicated odometer reading when I hit milemarker 100, and again when I cross the Utah line. The reason for the long distance is to give good resolution in the measurement. From these observations, it is then easy to calculate the proper correction factor. This varies every year according to tire wear.
Even with proper odometer correction, that still doesn't mean the exact same correction applies to the speedometer, even though it is close. Whenever driving past those radar trailers the cops put out, try to maintain a steady speed to get a good idea of your offset.
A GPS can also be used for both mileage and speed correction.