The basic operating principle of an automobile engine can be described as a simple air pump. When the piston nears the top, the air-fuel mixture is ignited, and the rapid expansion of the air forces the piston down, which rotates the engine crankshaft and ultimately moves the car. The air-fuel mixture will ignite and combust properly only if mixed in the proper ratio. When the car is in motion, it must be moved through the air. In hot weather, the lubricants in the engine and drivetrain run hotter and thinner, again requiring less power or fuel to maintain motion.
Tire rolling resistance is also less in hot weather compared to cold.
Betsy's simple question, however, becomes complicated when mechanisms other than the engine are introduced into the problem. Automobiles are equipped with various systems that we take for granted until they fail, and those systems affect gas mileage. Two systems that drastically increase fuel consumption are the electrical charging and air-conditioning systems. When the weather is hot and we turn on the air conditioner to keep cool, the electrical charging system is also operating the blowers that move the inside air and cool the engine's radiator. These loads on the engine outweigh the air density effects on the engine and those of the car moving through the air.