Roy Lewallen wrote:
In the admittedly very few looks I've had at mobile "shootout" results,
there seems to be more of a correlation between vehicle size and field
strength than antenna and field strength. This comes as no surprise,
since the vehicle is usually a comparable or even greater part of the
radiating system than the titular antenna, and its coupling to ground
has a large impact on the efficiency.

Roy Lewallen, W7EL

that seems quite plausible. A bigger vehicle essentially means a
physically larger antenna (think of the whole system as a dipole fed off
center, and a fan on one side but not the other.