Yes, a vehicle has multiple res. freqs. I don't recall the formula, but it has to do with the distance from the front most part of the vehicle, its furthest most part, the volume of the vehicle and something else or maybe not that much at all, but for wave path lengths of a sub and the vehicle to determine the vehicles res. freqs. They double in freq just as it is from one octave to another.
No offense, but you really have no idea what the resonant frequency of a vehicle is.
It is the frequency at which the vehicle, as a whole, resonates the most. There are various pieces of the vehicle that resonant at different freqs, like the plastic trim panels resonate at a diff freq than the engine block, but when you run sweeps -inside- your car, its "peak" is at one freq (or a small range). In the context of this discussion, the vehicle resonates the most only once. You may find multiple smaller peaks, but there is only one highest peak. And even this does not suggest the higher the freq, the more efficient the system will always be. Nor does it, going back to our original disagreement, alter the efficiency of the speaker in any way.
What you are describing, is the wavelength development cycle versus confining space. For example, your SUV may be 8 feet long from rear window to windshield, while the wavelength of a 60hz sound wave is approx 18 feet. The wave does not dully develop without reflections. This has lead to the common misconception that lower freqs are not fully audible. But it doesn't matter, because that's a complete different subject, and has nothing to do with resonant frequencies.
edit: resonant freq has nothing to with pathlegths of subs, it is affected by mass and shape. Something heavier resonates lower than something lighter, for example. Rigidity derived from shape can also affect resonance, as in a thin sheet of alum will resonate higher than the same sheet folded into a cube. Basic physics at work here.
