Internal Combustion Engines are ~25% efficient, give or take.
Although a gallon of gas has ~33 kWh of energy stored in it (see Energy Content Reference Points) - you can’t get 33kWh out of the engine you put the gas in.
Internal Combustion engines are well below 50% efficient, closer to 25% depending on where I’m reading it. So that 33.3kWh
From here:
Passenger car diesel engines have energy efficiency of up to 41% but more typically 30%, and petrol engines of up to 37.3%, but more typically 20%.
The car the engine is powering is itself contributing to lower effective energy output. The gas may hold 33kWh of energy, the engine may output 10kWh of that, but the energy of transferred to the wheels may be more like 8.5kWh.
Source
Internal combustion engine - Wikipedia