You'd want to compare net capacity as ton miles/gallon. Preferably as cargo rather than gross weight, though I'm using the latter for convenience.

Passenger cars net about 90 or so, for a high-efficiency vehicle, gross vehicular weight, at full capacity.

Assuming 140,000 lb GVWR and 7 mpg, I compute 490 ton-mpg. That's roughly equivalent to numbers given for rail ("move one ton of freight one mile on a gallon of fuel), and is likely high, though 100-250 ton-mpg seems likely.

This is not net cargo efficiency, where that rail number may come from. But gives a sense for numbers.