The tire changing poriton is under the ABS/brake section. Its kinda weird. What I did was select a tire size, then test drive it. My tires are damn near 35s. But the tire size that AE had most accurate was I think 33-34s? Cant remember. But all in all, I used my GPS to give me my speed, and then I matched it to my speedo and adjusted in AE accordingly.
I forgot what the speedo accuracy is anyways.. It can be off quite a bit I think.