As stated, the fraction you see is the fraction of the length of the wave, so, 1/4 wave length is 1/4th of the wave length (39.34' / 4 = 9.8'). Firestik
makes an antenna that works well for Jeep Wranglers. Since the ideal antenna mounting position is the metal top of the vehicle (the top acts as the ground plane for the antenna) and Wranglers have either a fiberglass or soft top, there is no ground plane.
Mounting an antenna to the side also affects the transmission so they have made a 'no ground plane' version that will allow you to better tune it to decrease the standing wave ratio (SWR) to a level that allows for the best TX/RX from your radio.