Google says 465 millimeters =~18". I doubt they parse the regulation so finely in Texas, but who knows.
I went looking for the [first] 4th Dymaxion that R. B. Fuller designed for Henry J. Kaiser. But the only picture I can find is this one:
...where it looks like it has radial gas engines. There exists an isometric drawing Bucky did in the 40s showing it with electric motors in each hub with a wheel on both ends of the electric motor's shaft.
Interestingly, when I asked Google to show me images, the second row showed the above, from when I posted it in
this thread from HydroJim titled Reverse Trike,
here.