Why gas consumption differs from highway to city



Why gas consumption differs from highway to city

In: Engineering

It’s because city driving is harder on your engine than driving on the highway. This is because your engine has to work harder to stop and start and change speeds frequently when you’re on city streets. This requires more gas (and burns oil faster), and therefore reduces gas mileage.

City traffic is, as a rule, less efficient than highway traffic because you’re constantly speeding up and slowing down in the city. Slowing down functionally wastes the kinetic energy of the vehicle, and requires you to burn extra fuel in order to accelerate again. Further, you end up burning fuel needlessly while waiting at the lights. All of that extraneous combustion adds up, and kills your fuel mileage.

On the highway; there’s no starting or stopping, so your car can just sit at it’s optimal speed/load and produce power.

Every time you brake you’re literally wasting fuel.

Energy from the fuel that got you moving is now being converted into heat in the brake discs, so you now need to use more to get you moving again.

Accelerating, particularly from a stop is very fuel inefficient.

City traffic is inevitably a lot more speeding up and braking than highway traffic, which is typically pretty steady speed, without much braking, at an efficient gear ratio with the engine sitting at an efficient RPM.

All that adds up to less fuel usage, overall, despite you moving faster.

City traffic is notoriously stop and go. For most cars, when you come to a complete stop your engine is still burning fuel but you’re not moving so its brutal on fuel economy. When you accelerate you burn a good deal of fuel to get moving, but then stopping again your brakes just take all that energy and turn it into heat to throw away.

Highways are more fuel efficient because you accelerate once (a lot of fuel for a bit) then cruise for a longgg time consuming a relatively low amount of fuel.

Highway speed is also more efficient for your engine than city speed. Your engine’s fuel consumption when cruising is its base fuel consumption + the fuel needed to overcome drag (rolling and air resistance). The base fuel consumption is always there regardless of the speed so traveling slowly results in poor fuel economy because even though drag is low your engine is burning a non-negligible amount of fuel just to keep spinning and your low speed doesn’t let you get many miles out of each gallon. As the speed picks up rolling resistance increases with speed and air resistance increases with speed^2 so there will be a point where things start getting worse, but modern cars are pretty good in the 55-75 range

Accelerating requires more power than maintaining speed; essentially when you maintain speed the power of the engine has reached equilibrium with friction and air resistance and other forces acting against it. If you want to go faster you have to upset the balance by supplying more power.

To that end city traffic is a lot of stop and go. If you’re always accelerating you never get to take full advantage of that equilibrium point. On top of that at the next stop light you have to shed all that energy you just got built up by hitting the brakes and start over again. All this keeps your car in the lower less efficient gears too, where it takes more engine spins per each turn of the tires.

It’s more fuel efficient to be moving, and at a steady speed. In the city, drivers accelerate more often from stops at stop lights, stop signs and turns, spend time idling at stop lights and drive thrus, pick-ups, etc.

Accelerating from a stop requires more gas for a variety of reasons having to do with gears and the efficiency of the engine at various rpms.

Highway driving benefits from momentum. When you let go of the gas the car still continues to roll for a while. So it doesn’t take a lot of energy to maintain a fast speed. But in the city you are constantly stopping at lights and stop signs, wasting that momentum.

Car engines are more efficient at certain speeds and those speeds are usually around 40-80mph

Also braking reduces efficiency decent high way travel means you aren’t using your brake and maintaining a constant speed. City travel meanwhile has stop lights, stop signs, turns, etc on top of having a normal speed of 10-30mph even if traffic is light.