Comment by dguest

I'm confused by where efficiency comes into this.

The efficiency is usually defined as the energy the car can use to move itself over the energy going in (electrical, or chemical for gas). If an ICE is 20% efficient then using 5 units means you get one unit out. Towing a trailer that doubles the load means you need 2 units out, which means you need 10 units in. So you get half the range either way.

The engine doesn't get more efficient just because you hook a trailer to it: it's still 20% efficient regardless.