Speed of best MPG
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Joined: Jan 2011
Posts: 42
True for our case. I recently did a 4 hour trip and due to traffic and fog, I was limited to 55-60 mph all the way there. A fillup before my return trip showed 38 mpg. Clear weather and traffic for the return trip, doing 70 -75 mph got me 31.8 mpg
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Joined: Jul 2012
Posts: 32
Generally
I cannot exactly remember which aerodynamics book I read that in, but I believe it was fluid mechanics with engineering applications. Ever since I read that I've read similar arguments that 90kph is close to the best efficiency. I remember many years ago a mechanic one time telling me that when you match your RPM position to your SPEED dial position you are getting the best efficiency. To this day I still don't know how true that is.
It is a general case but I'll bet you a tank of gas that 90kph is pretty close to optimal, above that aerodynamic drag affects your efficiency exponentially.
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If I agreed with you, we'd both be wrong.
Read:
The Ideal Vehicle Speed for Best Fuel Economy « Broken Secrets
Gas Mileage Tips - Driving More Efficiently
HowStuffWorks "What speed should I drive to get maximum fuel efficiency?"
Bottom line: a generalized "55-mph" speed is just untrue. Every car is different because every car has different variables that affect fuel economy. The only way to know for sure is to use a gauge like the ScanGauge to find out for sure.
Read:
The Ideal Vehicle Speed for Best Fuel Economy « Broken Secrets
The Ideal Vehicle Speed for Best Fuel Economy
by Chad Upton
February 25, 2010 at 12:01 am
The precise number varies by car and environmental conditions, but the sweet spot is generally between 40-60 mph (65-95 km/h). Most small and medium size cars get the best mileage at about 50 mph (80 km/h).
A lot of factors affect the fuel economy of your car. Some of them don’t vary much with speed, such as the resistance of the engine pumps and accessories. Other factors, like the size of the frontal area and the drag coefficient create increasing resistance with speed.
The faster you drive, the more energy is needed to overcome the aerodynamic resistance of the car. Up to 40 mph, that isn’t really even a factor. So, if you’re driving a box then your best fuel economy is likely closer to 40 mph than 50. If you’re driving a teardrop shaped eco-car, then you’re likely closer to 60 mph. Of course, engine size and other factors are involved too.
If you’re in a hurry, then going faster than 60 mph (95 km/h), presents new problems. Getting above this speed, your transmission is starting to run out of gears, so the engine has to keep turning faster and faster without getting any mechanical advantage from the transmission — that burns a lot of extra fuel. This also hurts your ability to overcome the exponentially increasing resistance — your engine can only turn so fast. But, if you’re in a small car then your fuel economy at 70 mph is likely still better than a large SUV traveling 60 mph.
If your car has a fuel economy display, be sure to take notice at many different speeds to find the best speed for your car. If your car does not have a fuel economy display and you do a lot of driving then it might be worth getting a device that will read your vehicle data and calculate fuel economy accurately. Try: Car Chip Pro ($75-85) or ScanGuage II ($160).
by Chad Upton
February 25, 2010 at 12:01 am
The precise number varies by car and environmental conditions, but the sweet spot is generally between 40-60 mph (65-95 km/h). Most small and medium size cars get the best mileage at about 50 mph (80 km/h).
A lot of factors affect the fuel economy of your car. Some of them don’t vary much with speed, such as the resistance of the engine pumps and accessories. Other factors, like the size of the frontal area and the drag coefficient create increasing resistance with speed.
The faster you drive, the more energy is needed to overcome the aerodynamic resistance of the car. Up to 40 mph, that isn’t really even a factor. So, if you’re driving a box then your best fuel economy is likely closer to 40 mph than 50. If you’re driving a teardrop shaped eco-car, then you’re likely closer to 60 mph. Of course, engine size and other factors are involved too.
If you’re in a hurry, then going faster than 60 mph (95 km/h), presents new problems. Getting above this speed, your transmission is starting to run out of gears, so the engine has to keep turning faster and faster without getting any mechanical advantage from the transmission — that burns a lot of extra fuel. This also hurts your ability to overcome the exponentially increasing resistance — your engine can only turn so fast. But, if you’re in a small car then your fuel economy at 70 mph is likely still better than a large SUV traveling 60 mph.
If your car has a fuel economy display, be sure to take notice at many different speeds to find the best speed for your car. If your car does not have a fuel economy display and you do a lot of driving then it might be worth getting a device that will read your vehicle data and calculate fuel economy accurately. Try: Car Chip Pro ($75-85) or ScanGuage II ($160).
Gas Mileage Tips - Driving More Efficiently
Observe the Speed Limit
While each vehicle reaches its optimal fuel economy at a different speed (or range of speeds), gas mileage usually decreases rapidly at speeds above 50 mph.
You can assume that each 5 mph you drive over 50 mph is like paying an additional $0.24 per gallon for gas.
Observing the speed limit is also safer.
While each vehicle reaches its optimal fuel economy at a different speed (or range of speeds), gas mileage usually decreases rapidly at speeds above 50 mph.
You can assume that each 5 mph you drive over 50 mph is like paying an additional $0.24 per gallon for gas.
Observing the speed limit is also safer.
HowStuffWorks "What speed should I drive to get maximum fuel efficiency?"
What speed should I drive to get maximum fuel efficiency?
This is actually a pretty complicated question. What you are asking is what constant speed will give the best mileage. We won't talk about stops and starts. We'll assume you are going on a very long highway trip and want to know what speed will give you the best mileage. We'll start by discussing how much power it takes to push the car down the road.
The power to push a car down the road varies with the speed the car is traveling. The power required follows an equation of the following form:
road load power = av + bv² + cv³
The letter v represents the velocity of the car, and the letters a, b and c represent three different constants:
- The a component comes mostly from the rolling resistance of the tires, and friction in the car's components, like drag from the brake pads, or friction in the wheel bearings.
- The b component also comes from friction in components, and from the rolling resistance in the tires. But it also comes from the power used by the various pumps in the car.
- The c component comes mostly from things that affect aerodynamic drag like the frontal area, drag coefficient and density of the air.
These constants will be different for every car. But the bottom line is, if you double your speed, this equation says that you will increase the power required by much more than double. A hypothetical medium sized SUV that requires 20 horsepower at 50 mph might require 100 horsepower at 100 mph.
You can also see from the equation that if the velocity v is 0, the power required is also 0. If the velocity is very small then the power required is also very small. So you might be thinking that you would get the best mileage at a really slow speed like 1 mph.
But there is something going on in the engine that eliminates this theory. If your car is going 0 mph your engine is still running. Just to keep the cylinders moving and the various fans, pumps and generators running consumes a certain amount of fuel. And depending on how many accessories (such as headlights and air conditioning) you have running, your car will use even more fuel.
So even when the car is sitting still it uses quite a lot of fuel. Cars get the very worst mileage at 0 mph; they use gasoline but don't cover any miles. When you put the car in drive and start moving at say 1 mph, the car uses only a tiny bit more fuel, because the road load is very small at 1 mph. At this speed the car uses about the same amount of fuel, but it went 1 mile in an hour. This represents a dramatic increase in mileage. Now if the car goes 2 mph, again it uses only a tiny bit more fuel, but goes twice as far. The mileage almost doubled!
Efficiency of an Engine
In effect the efficiency of the engine is improving. It uses a fixed amount of fuel to power itself and the accessories, and a variable amount of fuel depending on the power required to keep the car going at a given speed. So in terms of fuel used per mile, the faster the car goes, the better use we make of that fixed amount of fuel required.
This trend continues to a point. Eventually, that road load curve catches up with us. Once the speed gets up into the 40 mph range each 1 mph increase in speed represents a significant increase in power required. Eventually, the power required increases more than the efficiency of the engine improves. At this point the mileage starts dropping. Let's plug some speeds into our equation and see how a 1 mph increase from 2 to 3 mph compares with a 1 mph increase from 50 to 51 mph. To make things easy we'll assume a, b and c are all equal to 1.
Speed ...... Equation ............ Result
3 mph ...... 3+3²+3³ ................. 39
2 mph ...... 2+2²+2³ ................. 14
Power Increase ........................ 25
51 mph ...... 51+51²+51³ ... 135,303
50 mph ...... 50+50²+50³ ... 127,550
Power Increase ................... 7,753
You can see that the increase in power required to go from 50 to 51 mph is much greater than to go from 2 to 3 mph.
So, for most cars, the "sweet spot" on the speedometer is in the range of 40-60 mph. Cars with a higher road load will reach the sweet spot at a lower speed. Some of the main factors that determine the road load of the car are:
- Coefficient of drag. This is an indicator of how aerodynamic a car is due only to its shape. The most aerodynamic cars today have a drag coefficient that is about half that of some pickups and SUVs.
- Frontal area. This depends mostly on the size of the car. Big SUVs have more than double the frontal area of some small cars.
- Weight. This affects the amount of drag the tires put on the car. Big SUVs can weigh two to three times what the smallest cars weigh.
In general, smaller, lighter, more aerodynamic cars will get their best mileage at higher speeds. Bigger, heavier, less aerodynamic vehicles will get their best mileage at lower speeds.
If you drive your car in the "sweet spot" you will get the best possible mileage for that car. If you go faster or slower, the mileage will get worse, but the closer you drive to the sweet spot the better mileage you will get.
This is actually a pretty complicated question. What you are asking is what constant speed will give the best mileage. We won't talk about stops and starts. We'll assume you are going on a very long highway trip and want to know what speed will give you the best mileage. We'll start by discussing how much power it takes to push the car down the road.
The power to push a car down the road varies with the speed the car is traveling. The power required follows an equation of the following form:
road load power = av + bv² + cv³
The letter v represents the velocity of the car, and the letters a, b and c represent three different constants:
- The a component comes mostly from the rolling resistance of the tires, and friction in the car's components, like drag from the brake pads, or friction in the wheel bearings.
- The b component also comes from friction in components, and from the rolling resistance in the tires. But it also comes from the power used by the various pumps in the car.
- The c component comes mostly from things that affect aerodynamic drag like the frontal area, drag coefficient and density of the air.
These constants will be different for every car. But the bottom line is, if you double your speed, this equation says that you will increase the power required by much more than double. A hypothetical medium sized SUV that requires 20 horsepower at 50 mph might require 100 horsepower at 100 mph.
You can also see from the equation that if the velocity v is 0, the power required is also 0. If the velocity is very small then the power required is also very small. So you might be thinking that you would get the best mileage at a really slow speed like 1 mph.
But there is something going on in the engine that eliminates this theory. If your car is going 0 mph your engine is still running. Just to keep the cylinders moving and the various fans, pumps and generators running consumes a certain amount of fuel. And depending on how many accessories (such as headlights and air conditioning) you have running, your car will use even more fuel.
So even when the car is sitting still it uses quite a lot of fuel. Cars get the very worst mileage at 0 mph; they use gasoline but don't cover any miles. When you put the car in drive and start moving at say 1 mph, the car uses only a tiny bit more fuel, because the road load is very small at 1 mph. At this speed the car uses about the same amount of fuel, but it went 1 mile in an hour. This represents a dramatic increase in mileage. Now if the car goes 2 mph, again it uses only a tiny bit more fuel, but goes twice as far. The mileage almost doubled!
Efficiency of an Engine
In effect the efficiency of the engine is improving. It uses a fixed amount of fuel to power itself and the accessories, and a variable amount of fuel depending on the power required to keep the car going at a given speed. So in terms of fuel used per mile, the faster the car goes, the better use we make of that fixed amount of fuel required.
This trend continues to a point. Eventually, that road load curve catches up with us. Once the speed gets up into the 40 mph range each 1 mph increase in speed represents a significant increase in power required. Eventually, the power required increases more than the efficiency of the engine improves. At this point the mileage starts dropping. Let's plug some speeds into our equation and see how a 1 mph increase from 2 to 3 mph compares with a 1 mph increase from 50 to 51 mph. To make things easy we'll assume a, b and c are all equal to 1.
Speed ...... Equation ............ Result
3 mph ...... 3+3²+3³ ................. 39
2 mph ...... 2+2²+2³ ................. 14
Power Increase ........................ 25
51 mph ...... 51+51²+51³ ... 135,303
50 mph ...... 50+50²+50³ ... 127,550
Power Increase ................... 7,753
You can see that the increase in power required to go from 50 to 51 mph is much greater than to go from 2 to 3 mph.
So, for most cars, the "sweet spot" on the speedometer is in the range of 40-60 mph. Cars with a higher road load will reach the sweet spot at a lower speed. Some of the main factors that determine the road load of the car are:
- Coefficient of drag. This is an indicator of how aerodynamic a car is due only to its shape. The most aerodynamic cars today have a drag coefficient that is about half that of some pickups and SUVs.
- Frontal area. This depends mostly on the size of the car. Big SUVs have more than double the frontal area of some small cars.
- Weight. This affects the amount of drag the tires put on the car. Big SUVs can weigh two to three times what the smallest cars weigh.
In general, smaller, lighter, more aerodynamic cars will get their best mileage at higher speeds. Bigger, heavier, less aerodynamic vehicles will get their best mileage at lower speeds.
If you drive your car in the "sweet spot" you will get the best possible mileage for that car. If you go faster or slower, the mileage will get worse, but the closer you drive to the sweet spot the better mileage you will get.
Bottom line: a generalized "55-mph" speed is just untrue. Every car is different because every car has different variables that affect fuel economy. The only way to know for sure is to use a gauge like the ScanGauge to find out for sure.
Last edited by NovaResource; Jul 19, 2012 at 09:58 PM.
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Joined: Jul 2012
Posts: 5
Gas mileage for each vehicle varies greatly
My ET range chart I posted earlier has some strange results that I would not expect. Even though some of the high-speed results seem too high, there are factors that creep into it.
Wind resistance and other factors as pointed out by NovaResource are generally true for all cars and trucks. There are some speeds where the engine rpm/gearing ratio seem to enable higher than expected results.
My 2010 ET gets progressively worse mpg from 62mph on up to around the 72mph mark, then it suddenly "likes" it and the mph goes back up to as stated before about 34mpg. Increasing the speed beyond this, mpg progressively decrease until another weird sweet spot up around 79-82mph. Then my ET seems to get around 32-34mpg. Can't explain it. I drive a lot of highway miles under different conditions and when wind is against me mpg drops. Tail winds increases mpg by 2-5mpg depending on wind strength.
I have driven my ET enough to know that these sweet spots are reliable. I had one instance driving from Winnemucca, NV to Boise, Idaho where two yay-hoos driving a dually pickup thought "let's scare the California dude" or so it appeared. I was traveling at my 72mph sweet spot and had passed them. They were going I'd say about 60mph. Then suddenly about a minute later, they were right on my back-side. I mean almost fused to my bumper! A lone-some highway with almost no other cars, I hoped for a patrolman. Had to increase speed to 85mph to keep them off me. Point is that I got some of my best mileage ever, but suspected I was going to have BAD mpg reading. After a fill-up, I calculated 34mpg! This with occasional bursts of speed, and hill climbing. This incident is what got me to exploring the 80+ range in search of a sweet spot there.
Supporting NovaResource's info though is an intriguing result back in 1978 or so. A friend had an old pig of a 1961 Lincoln. A whale of a car, but if he set the speed-o to 56 mph, he got a solid 26-28mpg. Deviate above or below by too much and it dropped back to about 18! I always found that Ford motor cars on average always delivered better mpg than GMC and Dodge autos.
It would be nice if car makers allowed one to set your cruise control by RPM and have a memory with a few (4?) rpm's you could resume to. Just a thought.
Wind resistance and other factors as pointed out by NovaResource are generally true for all cars and trucks. There are some speeds where the engine rpm/gearing ratio seem to enable higher than expected results.
My 2010 ET gets progressively worse mpg from 62mph on up to around the 72mph mark, then it suddenly "likes" it and the mph goes back up to as stated before about 34mpg. Increasing the speed beyond this, mpg progressively decrease until another weird sweet spot up around 79-82mph. Then my ET seems to get around 32-34mpg. Can't explain it. I drive a lot of highway miles under different conditions and when wind is against me mpg drops. Tail winds increases mpg by 2-5mpg depending on wind strength.
I have driven my ET enough to know that these sweet spots are reliable. I had one instance driving from Winnemucca, NV to Boise, Idaho where two yay-hoos driving a dually pickup thought "let's scare the California dude" or so it appeared. I was traveling at my 72mph sweet spot and had passed them. They were going I'd say about 60mph. Then suddenly about a minute later, they were right on my back-side. I mean almost fused to my bumper! A lone-some highway with almost no other cars, I hoped for a patrolman. Had to increase speed to 85mph to keep them off me. Point is that I got some of my best mileage ever, but suspected I was going to have BAD mpg reading. After a fill-up, I calculated 34mpg! This with occasional bursts of speed, and hill climbing. This incident is what got me to exploring the 80+ range in search of a sweet spot there.
Supporting NovaResource's info though is an intriguing result back in 1978 or so. A friend had an old pig of a 1961 Lincoln. A whale of a car, but if he set the speed-o to 56 mph, he got a solid 26-28mpg. Deviate above or below by too much and it dropped back to about 18! I always found that Ford motor cars on average always delivered better mpg than GMC and Dodge autos.
It would be nice if car makers allowed one to set your cruise control by RPM and have a memory with a few (4?) rpm's you could resume to. Just a thought.
Last edited by solarjdp; Jul 20, 2012 at 10:46 AM.
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