There are limits; I don't know what they are, but there are too many people carrying too much fuel around for it to be as low as you suggest.
I take a 55 gallon drum in the back of my open pickup to be filled at the oil distributor. As far as I know, this is OK in Florida. I'm basing that on what I do know. I used to run a propane dispensing facility, and I had to know the rules about transporting propane. You can carry up to 90# in a closed vehicle (4-20# cylinders, 3-30# or 2-40#, for example). At 4.2 lbs/gal, that's about 20 gallons.
In an open vehicle, like a pickup or trailer, you can carry up to 200# (2-100# cylinders or any combination of smaller tanks) without having placards or shippling papers or such.
The cylinders have to be carried in an upright position and secured in some fashion.
Now, it was my responsibilty to notify those who were violating these rules, and to refuse to fill their cylinders if they didn't comply. For example, roofers would come by with 100# cylinders laying flat in the back of their pickups. I couldn't fill them until they stood them up and secured them, and they had to leave my facility that way. I couldn't stop them from laying them down once they were outside my gate, but I had a Duty to Warn when they were at my facility.
That Duty to Warn is extrememly important. If I failed to give the warning, or if I went ahead and refilled the cylinders that were not in compliance anywway, I could be held responsible for the results.
All that long-winded explanation was to say that I depend on my oil distributor. If it was illegal to transport a 55 gallon drum in my truck, he has a duty to warn me, and to refuse to fill it. Since he has done neither, I have to assume I'm legal.
Of course, I'm not stupid, so I didn't assume anything. I sked him, and he said, "You're OK." That's all I wanted to hear, so I didn't ask for specifics about the law. He also sold me the drum and helped me load and strap it in the pickup the first time...