Antenna radiation is either horizontally or vertically polarized. Mobile 2-way radio radiation is almost always vertically polarized. Mobile-to-mobile, station-to-station or mobile-to-station radiation polarization should be matched. If everybody else on the frequency and/or band has vertically polarized antennas, you should too, unless maximum performance is not important.
There is often confusion with regards to antenna installations when it comes to the rule of thumb regarding antenna length. In almost every case, a physically longer antenna mounted in the SAME location will return increased bandwidth and performance. However, that idea can be literally destroyed by someone who throws the "maximum height" wrench into the gears. If you are limited to 3-feet for whatever reason, that does not mean that you can/should install a 5-foor antenna and angle it at 40-degrees in order to keep the over all height at or below the 3-foot established height limitation. You will have more antenna length but you will no longer be vertically polarized. Furthermore, the relative position of the antenna to the ground plane may make it impossible to tune and for sure, will throw the ground-wave propagation pattern into the twilight zone. Long story short … do not try to compensate for height restrictions using angle adjustments that are greater than 15-degrees from vertical. But keep in mind, even at 15-degrees there will be changes in the propagation characteristics of the antenna, even if somewhat tolerable.
On occasion we hear people talking about seeing big trucks in the truckstops with their antenna(s) tilted forward. In most cases, drivers do this for two reasons. First of all, inasmuch as most of their driving is done between 55 and 75 miles per hour, their antennas are exposed to constant wind-drag during their periods of most use. So they tilt them slightly forward in order to have them as close to vertical as they can while at highway speeds. A second reason has to do with State and/or Federal height restrictions. Many of the mandatory weigh stations have a laser beam apparatus set at the maximum allowable height and they monitor every truck that pulls through the weigh station. If any parts of the truck, including the antenna tips, break the beam they will receive a fine for exceeding the maximum allowable height. So, some things that apply to Semi's do not apply to other vehicles. In many cases, because of the constant high speeds, a driver might replace 4-foot antennas with 5-foot antennas and increase the forward tilt enough to slip under the lasers. Again, they can get away with this because of the speed related wind drag and also the greater arc realized when the antennas physical length increases.
If you expect to experience maximum performance from any mobile, 2-way radio communications set-up, keep the antennas as vertical as possible.