I forget the exact power rating, but it's some voltage level at 3' is what the ruling from the FCC is.
I have a Neuros,
www.neurosaudio.com, and I've tried what you're trying to do, in an attempt to beat out the "homies" who have nice stereos, with 4 cars, two with nice stereos
. With all the cars next to each other and my Neuros in the middle, we were able to get the signal to all cars, but walking around between the cars pulled the frequency more than we had expected.
One friend actually ran down to the homies, several hundred feet away, and told them to tune in, I believe while something like Barenaked Ladies was playing, and he said that before they said "What is this <slighty more vulgar term of doody>?" he heard the signal coming through, but with way too much static for it to be of any use.
That being said, the transmitters you have will work, if you drive like maniacs. You'd want to get a transmitter that can go at least 500', probably more, because keep in mind that there will be plenty of stuff to screw up the signal, especially when driving.