Beg to differ,

Actually the body metal of a vehicle resting on rubber tires is not a "true"
ground.
However, it does offer a large resivour of metal which will "act" as a
ground for small electrical charges to
be dissipated.

A good example of this is the fuel filler ring on your car is "grounded" to
the car so that static
charge accumulated in the filling of the vehicle can dissipate into the
metal of the entire vehicle.

Filling a plastic 5 gallon gas can on the back of a pickup truck bed with a
plastic bedliner will not
allow any dissipation into the vehicles body metal and you will get a build
up of static charge.

The tires of a vehicle when filling the main tank with metal contact may
even offer more leakage to true ground than what you would get with the
above duraliner plastic gas can example.