If you stay close to the original size the dealer can update it. However you can figure it yourself if you use a tire size calculator. Just do a web search for "tire size calculator" and they usually will give a % +/- from the original size. Unless you go to some outrageous size you're probably going to be within 5 MPH of actual. With 225/75-16s on my 2008 my speedometer was showing about 4MPH slower than actual. That size is about as big as will fit on mine without a lift.
Your own experience and common sense should keep you pretty close to reality.
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