Well, with out more info on what you have there,
I'll take a stab at it.
If what you are using to mount it to the vehicle is
just a straight feed from the coax to the element,
you could use 19.25" of element length and it would
be a 1/4 wave. That is if your mag mount or direct
mno, or 3/8 thread base (whatever) has no coil in it.
The other part of the antenna (reflection) would
use your vehicle body as a ground plane to "work"
against. If said area is steel that is.
Going any longer with the "whip" gets quite tricky
as that's when you start making 5/8 wave elements
and they require coils to make the match for 50 ohlms.
SOOOOOOO many factors there to get the correct impedence I don't know how a person could homebrew such
a thing. Some people on bikes make 1/2 wave antennas but I think for that to work you need to feed it with a
1/2 wave of coax length.
Well that's my 2 cents,
73 Steve email@example.com