yea it will be off a little bit not a big deal. Mine is off about 4-5 mph. mine came with stock 245/75/16 and I went to a 265/75/16.
Now I have a 265/70/17 and they are almost identical with the 265/16, my speedomoter still reads about 4 mph slow. If it says I'm doing 70 then I am really doing 74