Most of the time, numerical values (tyre width, etc) are set using a scrollbar or +/- buttons along the scrollbar.
While it is convenient for gradual test purpose, very often I wished that I would able to simply type the numbers in the field (for instance because I did a test earlier and remember a specific value, etc), instead of exactly scrolling or clicking 10 times on + or - buttons.
It should be complicated to code. If you feel it could be confusing to some users, you can make it an option and not the default behavior. But for me, and I doubt I am alone, this possiblity to edit text fields that contains numbers would really be a plus.