Hi,
our vehicle models have a member function 'void SetInitFwdVel(double fwdVel)'. If you for example use
double forward_velocity = 40.0 * 0.44704; // convert mph into m/s
HMMW myCar;
...
myCar.SetInitFwdVel(forward_velocity);
...
myCar.Initialize();
The initial velocity is set now, but the vehicle need some time to reach steady state yet.