Agree with Tom.

A single bike on my Subaru is good for 2 mpg lost. 6 might get expensive.

As well, Thule, Yak as well as the car manufacturers often have a rooftop/rack weight limit of 150 lbs., which I'm sure is conservative, but by how much ?. I've no clue if all those Pro racing support vehicles use special mounting systems, but I'd be nervous at 80mph steady with that much load and wind resistance, which as BTW the TdF cars are not typically out on the highways all day.

Then there's nighttime security, how are you going to lock them ?, cables ?, etc... or are you hauling all the bikes into a hotel at night ?.

For these reasons I'd be building the trailer large enough to put the bikes in it.