Why do cars average a higher mpg on highways than in the city?
In: Engineering
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.
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