Basically, there is no set recalibration for larger tires. GM does not support larger tires than offered due to safety concerns.
Best thing to do is run the vehicle with a GPS to obtain what the speed of the vehicle actually is with the new tires.
This will serve two purposes; one, give you a factor to use for figuring fuel mileage, and two, let you know how much faster you are going than what is displayed on the speedo...helps to prevent tickets.
