<20% tank to wheel power (on the most efficent cars)
majority of output is heat, light and sound
not only is it wasteful to begin with, car manufactures waste EVEN MORE trying to reduce all the by-products
Since engines are so loud manufactures have to use catalytic converters, lots of piping, resonators, and mufflers to reduce the noise but results in the loss of even more engine power and added weight.
engines emit toxic crap as a by product so to reduce this, they need to use O2 sensors (2 or more of them) and catalytic converters (convert CO into CO2), but since cat's suck so bad as it is you need 2 of them in series robbing you of even more power.
as heat is a major product of any type of combustion, there is going to be tons of excess, so to prevent overheating cars need to use massive radiators and tons of fluid to cool off. but when installed your aero dynamics goes to **** since you are driving a brick wall around now, and the weight shoots up at least 100lbs-200lbs. all the while making you loose power since you have to pump that fluid around and overcome the drag.
now in order to burn gas you need O2 but since engines dont like to burn anything other than gas and air you have to filter the intake robbing you of power (airflow and volume)
burning any hydrocarbons to my knowledge has water as a by product, and by pushing water vapor through a bunch of steel it causes rust which in turn creates more maintenance and leaves crap all over the road
this is what i think about when i hear an Engine revving, what complete WASTE.
Gasoline is ridiculously power and energy dense to even make vehicles move forward even after all that loss, i think we could make use of gas in better ways but sadly this world is stupid and ran by greed.