102 inch ss whip, best quality 6 inch spring,, quality and well built mount,,, that is the best antenna,,,
Those numbers aren't exact, but they're certainly ball-park.
"If the ground plane is to small, you can compensate by making the antenna longer, and vice versa. What are the odds that the vehicle chassis is going to present the exact size needed for a groundplane..."
The first part, compensating for a 'too small groundplane' is not true. It doesn't work that way, sorry.
As for the size of a vehicle providing an adequate groundplane, most vehicles will provide an adequate one. Keep in mind that the vehicle -plus- the dirt/concrete under it make up that groundplane, not just the metal of the vehicle.
It's easier to think of the 'groundplane' as the 'other half' of a 1/2 wave antenna. That means it should be at least as long. Most vehicles will provide more than enough metal for that 9' (ball park number) not to mention the dirt under the vehicle.