Ive just discovered you should be able to use
phent_addcentralforce (me, vAhead);
instead of
phent_addforcelocal(me,vAhead,NULLVECTOR,my.pan);

I dont see how its working, because, from the manual, phent_addforcelocal only has THREE parameters
phent_addforcelocal(ENTITY* entity,VECTOR* vForce,VECTOR* vPos );
and yours has four !?!

Can you, just for my sanity, please try
Code:
vec_set(vAhead,vector((key_cuu-key_cud)*50000,0,0));   //feed keyboard into engine. 
vec_rotate(vAhead, my.pan);                            //rotate force to face tank direction
phent_addcentralforce(me, vAhead);                     // Now apply physics force in the directions it is facing....
Please?

"There is no fate but what WE make." - CEO Cyberdyne Systems Corp.
A8.30.5 Commercial