as you alluded to, you can't assume any parameter, you didn't (or can't) measure.
I didn't intend to say that. Sometimes parameters don't matter.
If I were modeling a high horizontal yagi, earth parameters would be irrelevant. Unless it was a particularly low impedance design, tubing conductivity wouldn't matter. What do I mean by they "don't matter?" I mean that the errors in gain and front to back and impedance prediction would be tiny. Very possibly swamped by construction realities. There are some types of antenna where NEC-2 predictions are better than anything I could ever hope to measure.
I don't think that's the case for mobile antennas. You need to measure SOMETHING about the parameters. But if you have a sophisticated enough model, you can back out the major parameter (total loss resistance) from impedance measurements. The only question is whether or not the model is sophisticated enough that all you need is a feed impedance measurement
to make it the model and real system match.
That is a "parameter measurement" but it's an indirect one that many people believe is useless. I'm not so sure. I think that feedpoint impedance compared to a good enough model
is actually a very useful measurement. It may be that for some (most?) hams, that would be a more reliable measurement than field strength. Field strength measurements have errors too, very possibly larger errors than impedance comparisons between measurement and good models.
I think the "impedance based" efficiency estimation fell out of favor based on a too-simple model of verticals over radial systems. That model was "the feed impedance of the lossless system stays the same as you add radials." That is false. But it didn't invalidate Efficency = Rrad/(Rrad+Rloss)! It just made it clear that you have to have a better way of knowing Rrad to use feed impedance measurements to calculate efficiency.
One last thing, that I want to set off down here so it doesn't get lost: WX7G is saying that mounting an antenna low on the car adversely affects its radiation resistance. This is consistent with your advice, Alan, that it's the "metal under the antenna, not to the side" that matters.
The vertical component of currents on the car flowing out of phase with the vertical antenna cancel in the far field with those from the antenna, reducing the radiation resistance. Those in phase reinforce those from the antenna in the far field. So a shorter antenna mounted higher might have more total radiation resistance.
It's just like how the radiation resistance of a ground plane increases as you slope the radials further and further downward. (Actually, like what happens going from UP-sloping radials to down-sloping ones.) That's really the main point of the original post, and I think it's probably right. Should we test it? Sure. Should we be skeptical? Sure. You always have to be skeptical of everything, especially your own beliefs. But let's discuss it directly.