I tried adding a comment, but it looks like I need a "50 reputation", so I'll add my comment as a post. You really don't give enough information about your setup, is this antenna for 2M, 6M 10M, 12M, 20M, etc.? The main function of a groundplane is to cancel stray currents on the coax shield, and provide an impedance to the the system that would not otherwise be reliable if your coax shield terminated and you simply had a 1/4λ length of conductor sticking out as your antenna; with that said, an uneven groundplane can cause directionality and cause your radiation pattern to be lopsided, as the electromagnetic field of the antenna radiator, wants to couple with the "ground". Anecdotally, when I ran 10M in my car with a bumper mounted antenna, the gain of the antenna noticeably favored the front of the car, in other words, in the direction of the groundplane. However, you can model a goundplane antenna that has 2- 1/4λ counterpoise elements,each 180 degrees apart from each other, you will see a radiation pattern that is nearly a perfect circle; add a 3rd element (each element then being 120 degrees apart) and you've got as perfect a circle in radiation as can be expected. Of course in a car, with the sheet metal being limited by the dimensions of the car, you may be forced to put up with a pattern that favors one vehicle direction over another; however, if you are in the UHF region, the affect on the pattern may be indecernable even if the antenna is mounted on a corner of the car.