If the truck still requires a human for some tasks (intra-city, fuelling, etc.) but can be rated to legally drive itself for most of the trip, then the human could sleep / eat / etc. during those times they're not required. This would presumably let them avoid minimum stationary rest times etc. since they could get a full rest of sleep while the truck drives itself (on sufficiently long journeys).

For owner-operators who live out of their vehicle anyway and are more concerned about getting work than sleeping in a hotel somewhere this could dramatically increase the volume of work they can take on (at least until fully autonomous trucks become widely available and humans are completely deprecated as truck drivers).

From that perspective it would be more efficient, not 90% but there would be significant gains.