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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The fuel efficiency do depend on the speed. It is in large part because the air resistance is proportional to the square of the speed. To stop air resistance from slowing you down is the main thing the engine do when you drive at a constant speed.
According to [https://afdc.energy.gov/conserve/behavior_techniques.html](https://afdc.energy.gov/conserve/behavior_techniques.html) the fuel efficiency drops rapidly and see the fuel efficiency also increase at very low speed. you can look at the worst case of you idel the car where fuel it used to keep the engine running and generate electricity but there is no motion. So a speed of 0 is the worst.
That said it is not efficient to go very slow because eighter. You can look at a graph like [like this](https://www.researchgate.net/profile/Sara-Hernandez/publication/261948625/figure/fig3/AS:286636043718656@1445350766029/Speed-fuel-consumption-curves-for-gasoline-passenger-car-by-engine-technology.png) and see the fuel efficiency also increase at very low speed. It looks to me that the min of the graph is at 65km/h but isis quite flat from 50km/h to 80km/h.
The exact number will depend on the car. If your car has a function to show current fuel usage that will be in liter/100km or miles per gallon. If it is liter/100km it should be as low as possible but as high as possible for miles per gallon. That value a fule usage compared to the distance not fuel usage per unit of time. So any effect of it taking longer to drive there is included in the value
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!
Air resistance is one of the main forces your engine is fighting against when driving your car. However, the relationship between your speed and drag isn’t linear. Doubling your speed causes air resistance to multiple by 4. The best thing you should think about to improve your gas mileage is driving smoothly and not accelerating or braking hard. Say there’s slower traffic ahead. Accelerating hard and braking hard when you get to it uses more gas than coasting there does. In my experience the best mpg I can get is when cruising at 40-50 mph on state highways where I almost never have to brake.
It’s very vehicle dependent. It’s related to engine efficiency at a given rpm, gearing, drivetrain efficiency, drivetrain harmonics, rolling resistance, aerodynamics, and how all of those interact. BUT. In general, your greatest efficiency will be at the lowest engine speed to provide enough power to keep you moving under light engine load in top gear. For most cars, that’s targeted at about 50-70 mph, depending mostly on gearing. This will vary depending on a number of factors. For instance, you can run a large displacement naturally aspirated engine at around 1800 rpm and make plenty of torque to keep the vehicle moving on the highway, so you can target that engine speed at that ground speed in top gear to theoretically yield greatest economy. Whereas you’d target a higher engine speed with a smaller engine where 1800 rpm is too low to provide adequate power to maintain speed. So in theory, slowish speed in highest gear is going to yield your best economy, as aerodynamic drag is the biggest factor, and it increases exponentially as speed increases. But you will get weird pockets of efficiency. For instance, I used to have a 2006 Mustang GT. It was definitely targeted to run about 65. That was about 1900 rpm in 5th gear. That would yield around 24 mpg highway. But if I ran it at 80, around 2300 rpm in 5th, it should have been worse due to higher engine speed and aerodynamic drag. But it ended up yielding 27-28 mpg consistently. Apparently the drivetrain harmonics were just happy and efficient there.
I have a brand new Seat Leon ST 1.5 petrol turbo manual 6 speed. Done some mpg tests on flat road using cruise control. I get best real-time readings at 30mph in 6th gear, which gives 70-80mpg @ around 900rpm (just above idle speed, I would never do this even on a slight hill). 60mph it drops to 50-55mpg, same gear, can’t remember rpms but obviously more. At 70mph it drops further to 40-45mpg. Air resistance sucks!
Most of the replies are spot on, but I’ll add that another fuel inefficiency is constant changing of speeds. Use of cruise control or maintaining constant RPM is the key to fuel efficiency. If you’re constantly speeding up/slowing down you can end up burning more fuel than if you just maintained a set speed.
If you push hard enough on the accelerator most automatic transmission cars will downshift and increate RPM so you speed up faster. There’s a threshold where you can still gain speed without causing the downshift. Learn it and you can improve gas mileage significantly.
It’s not that simple. There’s a speed that’s “ideal,” and the closer you are to it, the more gas you save.
Think about it like this. The car can generate energy from petrol, and it uses energy to move. The faster a car moves, the more energy it needs to overcome air resistance and friction with the road. So at slower speeds, you NEED less energy.
BUT. The amount of energy an engine produces is shaped kind of like a bell curve. At very low speeds, the car is not very good at making energy from 1 gallon of petrol, at very high speeds it’s also not very good. But there’s a sweet spot where it is really good.
You want to drive on average at a speed where the engine is very efficient and the car doesn’t waste energy pushing through the air and the road.
Electric cars kinda solve this problem, because they’re very efficient at all speeds. So for them, indeed going slower will save you the most power. For an ICE car, it’s a little more complicated and you will almost always use more energy at almost all speeds than an electric car.
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