You're confusing "earth ground" with the ground plane provided by your vehicle, which is much smaller and actually doesn't matter much in this case. Remember the "ground" around an antenna extends for several wavelengths, not just what's directly under it. Only way to determine the effect is with a field strength meter. It's a real eye-opener. A horizontal halo, loop, turnstile, Big Wheel or whatever mounted just 6" above the roof of a van produces most of its radiation to the sides, towards the horizon. I remember when Cushcraft did all their tests on the "Squalo" which came with 6" standoffs and suction cups to mount it to a vehicle roof, and that was before computer simulation and modeling. Very effective. Strong horizontally polarized signal just 15-20 degrees above the horizon, which is about where you'd want it to be, to launch above other nearby vehicles; not much "straight up" at all. They published those field trials with calibrated F/S readings back in about 1965 or 66.