I thought it sounded like a good setup, but I checked the internet and found
the URL below. Maybe it would help to spread a few ground wires out from
the base of the antenna to be sure of the counterpoise effect. I have an
SUV with one of the antennas in the middle of the roof. When talking to
local operators they tell me they can see the difference in signal when the
vehicle is pointed away or toward them. Must be a good counterpoise off the
roof.....

http://en.allexperts.com/q/Ham-Radio...-Roofing-1.htm