Why do cars average a higher mpg on highways than in the city?



Why do cars average a higher mpg on highways than in the city?

In: Engineering

In the city, there’s a ton of slowing down and speeding up, and you’re never really using any existing momentum to your advantage. On the highway, your car builds up momentum due to the speed, and you usually stay within a ±5mph range, rarely making any sharp turns, and therefore your engine doesn’t need to work as hard to keep you at the same speed.

City is like high intensity interval training where you slow down, stop, and accelerate a lot.

Highway is maintaining a steady pace for a longer time.

Which one is easier for you to do?

Depends on the car. At higher speeds it takes more energy to drive (need to push the air out of the way). At low speeds it takes much less. However in city driving you use the brakes frequently, and brakes convert energy to heat and throw it away (wasting fuel). Highway driving doesn’t use much brakes.

Second, when stopped your car still runs and uses gas. So waiting at a light is 0MPG. Also accelerating requires the engine runs through the inefficient speeds.

Hybrids and EVs change this, they don’t use gas when stopped and they don’t use brakes to stop (they have regenerative brakes), so the difference between highway and city is higher speeds need more energy to push the air which means they get better city MPG.

There are two major factors

1. Stop and go driving vs coasting – in the city you’re not just getting up to speed, driving for an hour, then decelerating. You’ll go from stopped to moving to stopped to moving several times over the course of your journey. Every time you stop you take the energy the car had and burn it off as heat(unless you have a hybrid with regenerative braking) and then have to add the same amount of energy back to the car to get back up to speed. This is why even relatively low speed highways can give you a big fuel economy boost

2. Engine base fuel consumption and drags – This is the one most people think of but it comes in multiple parts.

Your engine consumes a minimum amount of fuel per hour just to keep itself spinning. If your engine needs a quarter gallon per hour to idle then at 20 mph you can’t get better than 80 mpg even when coasting, but if you’re going 60 mph then it could be 240 mpg. The impact of this base fuel consumption on seen fuel consumption goes down as your speed goes up.

Next you’ve got rolling resistance which is determined by your tires. This resistance scales linearly with speed because its constant with distance. Each time your tire goes around you lose X energy so spinning the tire faster makes the power loss higher. This rolling resistance is what determines how much power your car needs to move at low speeds, you might need 5 HP at 20 mph and 10 at 40 mph. If we say that your car needs 0.1 gallons/hour per HP and combine it with the 0.25 gallons/hour up above then we see that you’ll get 26.6 mpg at 20 mph and 1.25 mpg at 40 mph. If you could run your car in a vacuum the fuel economy would increase the faster you got

But you don’t drive in a vacuum, you have to deal with air resistance which goes up with the square of the velocity. This is why you don’t end up with better fuel economy doing 90 mph than doing 60 mph, because that 50% increase in speed led to a 2.25x increase in air resistance which requires 3.375 as much power!

If you have good values for the engine’s base fuel consumption, tire rolling resistance, and drag coefficient you can plot it all out to find an optimal speed for your car to drive at, but if you move to a car with ecofriendly tires, or a worse drag profile then that number will change a bit. Its generally close to highway speed but there’s a fairly large window of optimal speeds.

It takes more energy to get up to speed than it does to move at a steady speed. And if you are stuck in traffic you are burning fuel without going anywhere.

It’s more efficient to go a steady speed than to accelerate with every stop sign, traffic light, turn. Also, fuel is wasted idling at stop lights and in traffic.

Sitting in heavy traffic where an engine is basically idling is 0 MPG. When going slow, the transmission is in a lower gear. This also makes for lower fuel economy because for all the power the engine produces much less distance is traveled.

On the highway you’re in a high or overdrive gear, so for the amount of power the engine is producing it’s covering a lot more distance. At some point aerodynamics factor in due to drag, but that still counts for a lot less than running an engine while covering less or no distance.

That’s why hybrids and electric vehicles are neat. Due to not needing to idle or significantly gear down an engine, their MPG (or equivalent) ratings between city and highway are the opposite of gasoline only vehicles.

In a perfect world, with no friction, something moving will stay moving at the same speed forever. This is a basic law of the universe. To change speed, you have to expend energy. So, stopping and starting takes more power than drifting along at a constant speed.