First and foremost, the mounting location is the most important aspect to performance. You must get as much of the antenna over the top of the vehicle as you can. That sounds very impressive. In a nutshell, no. There are some exagerated claims of performance from some companies.
Most people think that antennas mounted on vehicles are omnidirectional.
This is mostly true, but the radiation pattern is influenced greatly by the body of the vehicle. Well, we know that the body does not extend around the base on the antenna equally. Figure 2 shows how the radiation pattern of a whip is influenced by the body of the vehicle. As you can see, the shape of the pattern depends on where you mount the antenna. The pattern is the worst in directions where there is no metal body for a radial.