Hi,
Carl has given some good advice, and I just want to add a little.
On 2 meters, hams often use a "quarter wave ground plane". This is a quarter wavelength vertical (about 30 inches for FM) mounted perpendicular to the body of the car. A magnetic base is usually used. The car body forms the ground plane. You might consider buying a 2 meter 1/4 wave "mag mount" and replacing the 19 inch radiator with a 30 inch whip. Don't get a 5/8 antenna because there is a tapped loading coil in the base and it won't tune to the FM band.
I am curious as to why you want to go mobile with your transmitter.
Neil