What does the 18 stand for in a tire sized P235/75R-18?

Consider the transfer function for a first-order system g(s)=12(s+7)/(s^2+2s+9) for this system determine: a) poles and zeros of the function, b) the natural frequency and damping ratio, c) the peak value achieved in response to a unit step input, d) the peak time for a unit step input, e) the steady-state system response to a unit step input