Does a car get better gas mileage
in hot weather or cold?

It's a typical Monday. Betsy is running late, and, fortunately, has enough gasoline in the car to get to work. At quitting time, she starts the car, glances at the fuel gauge, and decides she has enough fuel to reach the gas station nearest her home. She's cruising at a good speed, but traffic becomes heavy and then slows to a snail's pace. It's a blistering hot day. As she anxiously eyes the big "E" on the fuel gauge, she ponders the effect of heat on fuel consumption. Does a car get better gas mileage in hot weather or cold? As she sits in traffic faced with the unhappy possibility that she may soon be walking the highway with a gas can in hand, let's get an SwRI Whizard to answer Betsy's question.

"Many projects are conducted at SwRI's Automotive Fleet Laboratory to answer simple questions such as Betsy's. Unfortunately, simple questions do not necessarily have simple answers. All other things being equal, a car will get better gas mileage in hot weather. Why? To keep things simple, let's not discuss the effects of barometric pressure and relative humidity changes. Instead, let's focus on air temperature (hot versus cold) and how this single factor affects the miles per gallon of a car.

"The basic operating principle of an automobile engine can be described as a simple air pump. Air and fuel are drawn into each cylinder as the piston moves down; then as the piston moves up, the air is compressed. 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. For gasoline, the average ratio for combustion (stoichiometric ratio) is 14:1; that is, 14 pounds of air is required to completely burn one pound of gasoline. With this in mind, hot air is less dense than cold air; therefore, the engine breathes less hot air per revolution and requires less fuel.

"When the car is in motion, it must be moved through the air. Hot (less dense) air provides less resistance to movement, so the engine does not use as much fuel to maintain motion 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. All industry-recognized test procedures used to measure fuel consumption characteristics specify that a vehicle be operated with the air conditioning system turned off. Therefore, the fuel economy advantage that results from the hot weather is thus negated if Betsy chooses to operate the air conditioner. And, as we all know, we can't control the weather.

"How can a driver get better gas mileage? An SAE (Society of Automotive Engineers) paper titled Nine Ways to Get Better Fuel Mileage offers the following suggestions: 1) use moderate speeds (the best cruising speed is 30-40 mph), 2) drive at a smooth, steady pace (as traffic permits), 3) accelerate slowly and allow automatic transmissions to upshift, 4) anticipate stops and minimize braking, 5) avoid prolonged warm-up idling, 6) limit extensive idling, 7) be sure parking brake is fully released, 8) minimize electrical loads and use of air conditioner, 9) consolidate short trips, plan routes in advance, and avoid heavy traffic. Plan ahead, Betsy!"

Thanks to this month's Whizard, Larry Eckhardt, manager of fleet and field evaluations in the Automotive Products and Emissions Research Division. Eckhardt specializes in conducting tests in commercial fleets and fleets operated by SwRI to evaluate the performance of lubricants, vehicle components, and systems.

The Lighter Side SwRI Home

March 25, 2013