When I first started driving I drove slow bc I thought it would save gas, then I started driving faster when a friend told me you use the same amount of gas whether you drive slow or fast (as long as it’s the same distance), you just would be driving fast for a shorter amt of time and driving slow for a longer amt of time, but at the end you burner thru the same amount of gas. Is this true?
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Cars are probably most efficient travelling around 50mph.
When you’re driving, your car is using energy to propel you forwards. At a very general level, there are a few things going on:
There is friction between your cars tires and the ground that needs to be overcome. This takes power. Think of the difference between sliding a puck around on asphalt as opposed to on ice or on a cushion of air. Friction is also needed to accelerate, decelerate, and turn too, so it’s not a bad thing here, but economy is at odds with grip. Friction with the road goes up in a linear fashion with speed. Twice the speed is twice as much friction.
There is resistance from air that your car needs to push through. The more resistance, the more power it takes, the more fuel you use. The thing about wind resistance is that it doesn’t go up in a linear fashion, it I creases exponentially. If you double your speed it is 4x as much wind resistance. That has a huge effect on mileage. Wind does too – driving in to a 20mph headwind will use more gas than with a tailwind. This is the primary reason trucks are so much less efficient on the highway than cars.
The engine itself is inefficient. When I say it is inefficient, I mean that it only turns about 25-35% of the power in gasoline and turns it in to propulsion. When it is producing very low amounts of power, it is almost like it is idling. Engines tend to be efficient with some load on them. Different engines may be more efficient in different power and RPM ranges. Power is also lost in the transmission.
The car’s weight directly affects how much power it takes to accelerate… But it is less important than wind resistance as it is linear, just like friction.
At 50mph on level ground, a car probably only needs about 10-20 horsepower to maintain speed. At 100 miles per hour, the car will need more like 70-80 horsepower. Even if the engine is somewhat more efficient when making more power, that efficiency is more than offset by the amount of power required to propel the vehie forwards.
I don’t know how much power an engine puts out at idle, but if the engine is making 5 horsepower at idle, uses 10 horsepower at 25mph, and 15 at 50mph, you can see how 50 would be much more efficient than a very slow speed. And as you go faster you can see that the required output is significantly higher as a proportion, too!
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