Fs - Ff = Fd

where

Fs = W*sin(theta), Ff=mu*Fn=mu*W*cos(theta) and Fd=(Cd*A*p*v^2)/2

hence

sin(theta) - mu*cos(theta)=(Cd*A*p*v^2)/2*W

By using the initial values of the simulation (theta=20, mu=0.1, W=60 and Cd*A*p=0.84), and solving the equation for v, we find that vmax = 19.4 m/s (53.9 km/h) where the simulation predicts 63.8 km/h.