when fitting a cb in any motor car / van / lorry for all round reception as mavericktc says place aerial in the middle of the roof but if you want to transmit directionaly say you want to transmit mainly in front of you place your aerial at the rear your motor roof will act as a ground plane and pull your signal forwards same as if you want most of your signal going behind you place aerial at the front so if you want most of your signal going in one direction place your aerial on the opersite side of the roof most people want Omni direction so yes in the middle of roof is best
most aerials have a loading coil thus making the overall length smaller for 27 mgs with out a loading coil you will need a wipe of just under 9 ft but remember when you transmite on any aerial you don't transmit from the top you actually transmit from about a foot above the loading coil all the rest of the aerial is just to balance the signal it runs up and down the aerial and jumps of about 1 ft above the coil most cb aerials are base loaded but at one time you could get centre loaded coils but not seen any centre loaded aerials for a long time