It is more complicated with ICE though because the efficiency changes with load amount, and what power output an ICE is most efficient in can be designed with towing in mind. Ive seen 90s trucks that got 18 MPG empty on a good day, but then when loaded with twice or more the vehicle's curb weight on a trailer merely drop to 17 MPG.

Generally this is only true for ICE with an abundance of extra power to start with, but that is basically every truck and most other cars for the last 25 years.