In the United States, if you need more than about 20 tons of freight moved, you use the railroad not the highway. “Road trains” aren’t a thing here.

Regardless, I’m not quite sure what you’re trying to illustrate - your example of a “streamlined vehicle” undoubtedly consumes a lot more diesel than your example of a vehicle which is not streamlined. Isn’t that antithetical?

In absolute terms, yes. A semi truck consumes more fuel than a pickup truck. So does a diesel locomotive. The point is the difference in aerodynamic drag with and without the trailer.

Aerodynamically, a semi truck is the same shape with and without a trailer. Somewhat streamlined nose, flat sides, flat rear. With the trailer attached, that flat rear is just 50 feet further away. A semi truck without a trailer will make better fuel economy because it doesn’t have to accelerate the additional weight or overcome the friction in the additional axles, but the cab has already taken most of the aerodynamic punishment it can.

With a non-streamlined pickup truck, the additional wheel friction and acceleration energy are factors, but you’re also functionally doing this:

So a pickup truck sees a much bigger punishment to fuel economy when you hitch up a trailer than a semi does.

Yes, but a “truck” with a 4 ton towing capacity obviously needs motors with more torque than those on a tesla.

Not necessarily. My S10’s 4.3L V6 engine makes 200 horsepower and 200 foot pounds of torque. It’s rated to tow 7600 pounds, with the addition of a transmission cooler. A Tesla Model Y is rated at 397 horsepower and 389 foot pounds of torque, it is rated to tow 3500 pounds. A lot more factors than horsepower and torque go into a tow rating. The strength of the frame at the hitch, a height sensing proportioning valve, the duty cycle ratings of the power train to name a few. It’s not about “Can this car get this weight moving” it’s “can this car perform a safe emergency stop 9 hours into a 10 hour journey towing this weight?”

As little as 5 to 10% on the highway, not particularly significant.

See that’s the funny thing about EVs. ICEVs get better fuel economy on the highway because the constant aerodynamic drag at cruising speed is less than all the braking you do in the city. EVs are the other way up, because of regenerative braking they get a lot of their energy back when braking, but you can’t get aerodynamic drag back. And that’s without the additional drag of a trailer.