'M.M.',
The length of the antenna isn't going to make a lot of
difference, but it's height will, sort of. Extending the
length of your car's antenna isn't the answer (no matter
if it's for AM or FM).
If you could raise the height of the antenna to 100 or
more feet, you'd certainly hear more stations because you
will have extended the antenna's 'horizon' further. 'Course,
that 100 feet height would make driving down the highway
kinda difficult...
There are no 'magic' fixes to extent radio reception in
the average car's AM/FM radio. 'Practical' isn't the same
as what would be 'best', and your 'better-half' probably
wouldn't put up with it anyway...
'Doc