How much clearance????

13'6 is bridge hieght....unless posted differently. Be careful,go slow get out and look .Watch bridges that cross a valley.

 

You can run a car within about 8" of the cab usually.

 

I'm replying the obvious but...watch out for turns in a dip. That'll decrease your space real quick. I bought a brand new Dodge/Cummins dually and took it to the Take-3 factory in Texas to get fitted up. Figured I'd load it for the ride back home. Don't ask me how I know about the above warning. It was just a little scuff but it could've been much worse. And that tail swing...

 

I never hauled cars. I did notice a tunnel creates a vacume and the cars lift up a few inches. Don't know where your going, but keep it in mind.

 

I'm almost certain the OP is inquiring about cab to trailer/load/overhang clearance in a turn rather than overhead vertical clearances. In 99.9% of cases, a wedge operation won't have to be very concerned about regular overhead clearance until they get off in the weeds, trees, special circumstances, high-top conversions, etc. Those flip out extensions can gouge in a turn and dip situation if one is not careful.

As for tunnels creating a vacuum and lifting cars up a "few inches"...I'mma leave that one alone but look forward to some of the less timid observers.

 

I'm watching in a tunnel. I got a step deck on. The cars rise up. I don't know why. Lift from some force in the tunnel, is all I got.

 

I'd keep them Atleast 1' away from the back of the cab. 1.5' comfortably. My opinion is 8" is asking for trouble in dips and turns as someone already posted about.