BBS **** wheel diameter settings

Hi to the group,
A "stupid" question please. I have a BBS -02 . I have set the wheel diameter to 26" in the program to match my wheel size, but what please sets the tyre thickness ? or does the program assume a certain tyre size. If so what is it ? I'm running a 1 3/4" tyre so in theory that could be 26" plus 2 X 1 3/4" = 29 1/2 " effective rolling diameter.

Regards Bob

The wheel diameter settings in my BBS do not give acccurate speeds. I just use a GPS and change up or down as necessary.