If you mount an omni horizontally, you will have very limited range, as omni antennas are vertically polarized with a horizontal beam eminating 360 degrees with a virtical beam width of approximately 25-30 degrees.
Without getting too technical, you will basically have great range directly to the left and right of the vehicle, but not forward and in back of the car. If you mount it running across the car, you will have coverage to the front and read, but not the sides. I would recommend instead you buy one of the low profile ceiling mount style antenna (usually about 3.5dbi gain) and mount it in the center of you rack on the roof of the vehicle so you will have 360 degree coverage, and about a 35-45 degree verticle beam width as well.
Hope this helps