The origin for each wheel in MED should be exactly at the point where the axel connects to the wheel(on outside of wheel, axel side). If that doesnt fix it be sure that the tires and your chassis are all set within the same group - "phent_setgroup(my,2);" apply that for all wheels and chassis. Be sure your wheels are using spherical collision, it does not matter what your chassis is using. Finally, be sure that when your creating your wheels their verticie positions are right.

Heres a quick example:

Code:

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//Put this in your chassis action after your chassis is applyed to physics engine
vec_for_vertex(temp,my,952); ent_create(mdl_tyrefr,temp,FRInit);
 vec_for_vertex(temp,my,953); ent_create(mdl_tyrefl,temp,FLInit);
 vec_for_vertex(temp,my,950); ent_create(mdl_tyrebl,temp,RRInit);
 vec_for_vertex(temp,my,951); ent_create(mdl_tyrebr,temp,RLInit);

//An example of the wheels action, it is applied via its "my.x" position, so be sure your origin is right.
action FLInit
{
 phent_settype(my, PH_RIGID, PH_sphere);
 phent_setmass(my, massWheel, PH_sphere);
 phent_setfriction(my, fricWheel);
 phent_setdamping(my, 0,dampWheel);
 phent_setelasticity(my, 0, 100); // bounciness
 phent_setgroup(my, 2);
 my.FL_wheel_ID= phcon_add(PH_WHEEL, you, my);
 phcon_setparams1(my.FL_wheel_ID, my.x, vecUp, vecRight);
 phcon_setparams2(my.FL_wheel_ID, vector(0,0,0), nullvector, vector(suspensionERP, suspensionCFM,0));
}
 

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P.S - I would reccomend creating the wheels through script as shown above instead of placing them beside the chassis in WED. That way, its precise and instant. Just be sure to place a vert's at the end of each axel(in the middle...) and create the wheels according to the verticie positions.