Quick facts

If you have a 200 hp, 2000 pound car, it will reach 60 mph in 1 seconds. Why, because 60mph is 88 feet per second, and a horsepower is 440 ft-pounds per second, and some math. If your car takes 8 seconds when you floor it to get to 60 mph, you effectively have a 25 horsepower car. Well um Torque and... Nope, 200 horsepower would by definition create enough torque to do 200 horsepower worth of work.

Cartoon science films from grade school are never wrong are they?
The average pressure inside car engines to maintain 30 mph is under 2 atmospheres, or 30 pounds per square inch.  At maximum power a good engine can manage an average of about 9 atmospheres, or 135 PSI, an average shop compressor you could buy. It is not a tiny explosion. Your car is powered by tiny hot air balloons, is more accurate.

Well, an engine advertised as 200 HP is 200 horsepower at 6000 rpm, which is a speed your car probably won't LET the engine run at. It will wear out in a hurry for one reason, you don't need 200 horsepower for another. Its just marketing. A big part of this is that it takes time for your gas engine to build up speed. Today's engines idle at 1000 rpm, which means they produce 0 horsepower, while consuming 5 to 15 horsepower of heat.

But they can't say its well 10 hp at a bit faster than idle and then it goes up and up... They tell a standardized MAXIMUM horsepower rating. Is it realistic? No, but every car maker has the same guidelines, so it is useful to compare between gas and diesel engines from different makers.

Electric motors can instantly (compared to gas engines) deliver their rated horsepower. That's why a Big clunky Lexus hybrid with a 300 horsepower engine gets zero to 60 faster than a sleek Ford Mustang with a 400 horsepower engine. The hybrid's electric motor assist is instant, cutting seconds of the time to get going. (Even with the electric motor horsepower added in, the Mustang's horsepower to weight is still better, half the wind area, and yet the Mustang still accelerates slower.)

Highway miles are dominated by wind resistance, which is cubic with speed. So highway speed take (70/30) cubed, or about 12 times more power. It gets more distance per time too, so when you figure Wind Energy per mile, highway miles take 5 times more energy than city miles. So why is city mileage worse? Because Gas engines consume many horsepower to idle. It takes about 5 horsepower just to idle a small engine (2 liter), and it scales up with engine size.

Hybrids don't BEGIN to fix the difference. Their primary advantage is by turning the engine off when stopped.

If the difference was "fixed", city mileage should be about 3 times highway mileage. It takes about 1 unit of energy for rolling resistance and 1 for wind resistance at 30 mph. It takes about 1 rolling and 5 for wind at 70 mph. If you car gets 30 mpg highway, it should get 90 mpg city. (And does not because of how much power it consumes doing nothing- idling.)

Engines waste 90% to 96% of their energy at city speeds.

A gallon of gas is 114,100 btu or 33.44 Kilowatt-hours or 44.8 horsepower-hours. A small car can maintain 30 mph with about 2 horsepower, so a gallon could provide 2 horsepower for 22 hours, or 660 miles.

Car engines are pretty much all the same. They last got "more efficient" when they raised compression ratios from about 7 to about 10, helping highway mileage and hurting city mileage. Why does car mileage go up with each new years model? The car manufacturers tweak wind resistance by making cars more aerodynamic. They get better mileage because the body changes. Engine technology is effectively frozen in time, and 100 year out of date.

Currently, horsepower ratings are usually "brake horsepower" or BHP. That has nothing to do with car brakes. The engine is mounted along with all its parts, like a water pump, and measured on a stand. The brake horsepower is usually about 10% less than the theoretical horsepower it should generate based on the average pressure inside the cylinders. So that includes all internal losses to friction, dissipation of heat, and load of things like the water pump. What internal combustion engines put out is exactly what the calculations from air pressure say they should. Engine friction is measurable, but not significant in determining gas economy.

Most of the energy of your gasoline is lost "heat leaking", which means radiated directly to the air by the radiator and engine block.

A typical home has a 200 amp main breaker at 220 volts. This means it can serve a maximum of about 180 amps, or 40 kilowatts. Typical electric bills are from $30 to $300, which corresponds to 10 to 100 kilowatt-hours per day. In horsepower this is about 54 horsepower maximum, and 13 to 130 horsepower-hours per day. 130 would be heating or cooling electrically all day.

A small car, 150 horsepower or 120 kilowatts, has three times the MAXIMUM power consumption of your home. If you drive it 2 hours a day, that's about 60 horsepower-hours. Drive it all day, its much more than your home could consume.