20 mph Speed Limits Do Save Petrol
Discussion
Driving more slowly will save drivers up to £500 a year in fuel costs, according to a study, which reveals that the most efficient speed is much lower than most people think.
Car manufacturers suggest that the optimum speed for fuel efficiency is between 50mph and 60mph and a recent survey found that two thirds of drivers believe this to be the case. But the study, commissioned by What Car? magazine and based on five cars of different sizes ranging from a 1 litre Toyota Aygo to a 2.2 litre Land Rover Freelander, found that the most efficient speed was below 40mph for all five and as low as 20mph for two.
Full story here:
http://driving.timesonline.co.uk/tol/life_and_styl...
Car manufacturers suggest that the optimum speed for fuel efficiency is between 50mph and 60mph and a recent survey found that two thirds of drivers believe this to be the case. But the study, commissioned by What Car? magazine and based on five cars of different sizes ranging from a 1 litre Toyota Aygo to a 2.2 litre Land Rover Freelander, found that the most efficient speed was below 40mph for all five and as low as 20mph for two.
Full story here:
http://driving.timesonline.co.uk/tol/life_and_styl...
[quote]The average driver travelling at 90mph on a motorway will spend £1.20 more on fuel every eight minutes than a driver travelling at 70mph. The 90mph driver will have travelled farther in that time but will still be spending 40 per cent more per mile than the 70mph driver.
[/quote]
That just can't be right, I've tried those two on full tanks of petrol and not noticed even 2% difference. Perhaps they didn't test a Mondeo.
[/quote]
That just can't be right, I've tried those two on full tanks of petrol and not noticed even 2% difference. Perhaps they didn't test a Mondeo.
Whilst designing my current car, I worked out that a typical car is most efficient at around 30 MPH. The actual peak varies according to all sorts of factors, not least your engine, gear ratios, car weight and aerodynamics. The curve is pretty wide and any speed between 20 and 40 is good. Above 50 MPH, you'll lose 2% for every MPH you increase your speed, so if your idea of a 60 limit is more like 70, you'll be using perhaps 20% more fuel per mile. Above 70 or 80, though, the curve flattens out a little.
Codswallop!
I cannot imagine a C4 1.6 diesel being able to do this at 20mph. Perhaps on over-run, but would suggest to do so on a regular basis would cause undue damage to the engine.
Article said:
The study, by Peter De Nayer, a former AA fuel efficiency expert, involved fitting cars with a fuel flow meter and testing them at Millbrook proving ground in Bedfordshire. He found that a Citroën C4 1.6 diesel achieved 99.6mpg at 20mph but only 29.3mpg at 90mph
Any vehicle is going to be at peak fuel efficiency when in top gear, with an engine speed/RPM that it is comfortable with without being laboured. I cannot imagine a C4 1.6 diesel being able to do this at 20mph. Perhaps on over-run, but would suggest to do so on a regular basis would cause undue damage to the engine.
Why do you think top gear is in any way important? It will make no difference to the efficiency at all. All gears are equally lossy. On old fashioned 4 speed gearboxes, 4th was "straight through" and so a little more efficient but modern gearboxes don't do that any more.
The prime factor in this is the amount of power (energy per second) needed to push the car at a given speed versus the distance travelled per second, with a consideration for the amount of fuel burned to make that power. Apart from a few oddities at speeds below maybe 5 MPH, power demand rises roughly linearity at first when rolling resistance fat exceeds aerodynamic drag. It then rises faster as the two become about the same (I read a survey that suggests an average of 47 MPH) then it rises much faster as aerodynamics dominate. The POWER required to overcome aerodynamic drag increases with the cube of road speed. [For pedantic types, the FORCE required increases with the square of speed].
An engine's efficiency varies with load and revs. The sweet spot is often between 1/3 and 1/2 of the maximum revs at maybe 70% or 80% throttle. Choosing the right gear for the speed affects MPG a bit by moving the engine to a more efficient spot. Being in top gear can be terrible if you're going too slow.
I'm afraid your codswallop is nothing more than received wisdom, rather than a reflection of the truth. Sorry.
The prime factor in this is the amount of power (energy per second) needed to push the car at a given speed versus the distance travelled per second, with a consideration for the amount of fuel burned to make that power. Apart from a few oddities at speeds below maybe 5 MPH, power demand rises roughly linearity at first when rolling resistance fat exceeds aerodynamic drag. It then rises faster as the two become about the same (I read a survey that suggests an average of 47 MPH) then it rises much faster as aerodynamics dominate. The POWER required to overcome aerodynamic drag increases with the cube of road speed. [For pedantic types, the FORCE required increases with the square of speed].
An engine's efficiency varies with load and revs. The sweet spot is often between 1/3 and 1/2 of the maximum revs at maybe 70% or 80% throttle. Choosing the right gear for the speed affects MPG a bit by moving the engine to a more efficient spot. Being in top gear can be terrible if you're going too slow.
I'm afraid your codswallop is nothing more than received wisdom, rather than a reflection of the truth. Sorry.
robinhood21 said:
Codswallop!
I cannot imagine a C4 1.6 diesel being able to do this at 20mph. Perhaps on over-run, but would suggest to do so on a regular basis would cause undue damage to the engine.
Article said:
The study, by Peter De Nayer, a former AA fuel efficiency expert, involved fitting cars with a fuel flow meter and testing them at Millbrook proving ground in Bedfordshire. He found that a Citroën C4 1.6 diesel achieved 99.6mpg at 20mph but only 29.3mpg at 90mph
Any vehicle is going to be at peak fuel efficiency when in top gear, with an engine speed/RPM that it is comfortable with without being laboured. I cannot imagine a C4 1.6 diesel being able to do this at 20mph. Perhaps on over-run, but would suggest to do so on a regular basis would cause undue damage to the engine.
Edited by smart51 on Sunday 15th June 22:22
smart51 said:
Why do you think top gear is in any way important? It will make no difference to the efficiency at all. All gears are equally lossy. On old fashioned 4 speed gearboxes, 4th was "straight through" and so a little more efficient but modern gearboxes don't do that any more.
The prime factor in this is the amount of power (energy per second) needed to push the car at a given speed versus the distance travelled per second, with a consideration for the amount of fuel burned to make that power. Apart from a few oddities at speeds below maybe 5 MPH, power demand rises roughly linearity at first when rolling resistance fat exceeds aerodynamic drag. It then rises faster as the two become about the same (I read a survey that suggests an average of 47 MPH) then it rises much faster as aerodynamics dominate. The POWER required to overcome aerodynamic drag increases with the cube of road speed. [For pedantic types, the FORCE required increases with the square of speed].
An engine's efficiency varies with load and revs. The sweet spot is often between 1/3 and 1/2 of the maximum revs at maybe 70% or 80% throttle. Choosing the right gear for the speed affects MPG a bit by moving the engine to a more efficient spot. Being in top gear can be terrible if you're going too slow.
I'm afraid your codswallop is nothing more than received wisdom, rather than a reflection of the truth. Sorry.
Perhaps you would like to do an experiment: Take any vehicle, drive it in all gears, and report back on in which gear it is most economical?The prime factor in this is the amount of power (energy per second) needed to push the car at a given speed versus the distance travelled per second, with a consideration for the amount of fuel burned to make that power. Apart from a few oddities at speeds below maybe 5 MPH, power demand rises roughly linearity at first when rolling resistance fat exceeds aerodynamic drag. It then rises faster as the two become about the same (I read a survey that suggests an average of 47 MPH) then it rises much faster as aerodynamics dominate. The POWER required to overcome aerodynamic drag increases with the cube of road speed. [For pedantic types, the FORCE required increases with the square of speed].
An engine's efficiency varies with load and revs. The sweet spot is often between 1/3 and 1/2 of the maximum revs at maybe 70% or 80% throttle. Choosing the right gear for the speed affects MPG a bit by moving the engine to a more efficient spot. Being in top gear can be terrible if you're going too slow.
I'm afraid your codswallop is nothing more than received wisdom, rather than a reflection of the truth. Sorry.
Edited by smart51 on Sunday 15th June 22:22
I'd love to. Anyone want to lend me a suitably programmed laptop or a Tech2 and a vauxhall?
Here is a graph drawn up from typical rolling resistance and aerodynamic data, with brake specific fuel consumption data supplied by Lombardini, ising gear ratios of 5, 9, 14, 18 and 22 MPH per 1000 RPM.
As you can see. Each gear gives the best MPG at its own range of speed, when the engine is at its best. Top gear is best from about 50 MPH upwards but is the worst at 20 MPH, not unexpectedly. 3rd gear is best from 30 to 40 but the peak in this example is at 20 MPH in second, Probably because the lombardini engine is fairly low revving, like a diesel.
Here is a graph drawn up from typical rolling resistance and aerodynamic data, with brake specific fuel consumption data supplied by Lombardini, ising gear ratios of 5, 9, 14, 18 and 22 MPH per 1000 RPM.
As you can see. Each gear gives the best MPG at its own range of speed, when the engine is at its best. Top gear is best from about 50 MPH upwards but is the worst at 20 MPH, not unexpectedly. 3rd gear is best from 30 to 40 but the peak in this example is at 20 MPH in second, Probably because the lombardini engine is fairly low revving, like a diesel.
Edited by smart51 on Monday 16th June 09:40
Edited by smart51 on Monday 16th June 16:10
smart51 said:
Why do you think top gear is in any way important? It will make no difference to the efficiency at all. All gears are equally lossy
And all engines behave differently which makes your statement ridiculous in the context of 20mph efficiency. There are many different cars and many different engines. Enforcing a blanket 20mph limit stating its the most fuel efficient speed can only be so for certain vehicles. In such a study it's the percentage of those most fuel efficient that would support such criteria - or not, as your below comment appears to suggest also.smart51 said:
but the peak in this example is at 20 MPH in second, Probably because the lombardini engine is fairly low revving, like a diesel
So what happens at 20mph in 2nd gear in a fairly high revving engine? Or how about a fairly low revving long stroke engine at 20mph in 5th gear? Any study needs to make comparison based on real world data. Plucking data from suitable engine\transmission combinations that suit a particular argument\debate\study\whatever is floored. It's that simple and not worthy of debate until all scenarios are looked into.
First of all, read what I ACTUALLY wrote. I didn't say that 20 MPH was the ideal speed for all cars, The guy who started this thread was quoting a report which said 2 of the cars tested are best at 20. MY thoughts are that most cars are most efficient in the 30 to 40 MPH range. This is based on some research and computerised vehicle efficiency models. I don't advocate a 20 MPH limit because of this. In fact, I have been in discussions with a pressure group on 20 MPH speed limits trying to convince them that they are wrong. If you want to save fuel (and if the forecourts near you have sold out then you might) keeping your speed down is a good idea, but 40 MPH would be a much better idea than 20 for the reasons that I have stated.
Perhaps you should read what people actually say before making comments, you might not look like such an idiot then.
Any study needs to make comparison based on real world data. Plucking data from suitable engine\transmission combinations that suit a particular argument\debate\study\whatever is floored. It's that simple and not worthy of debate until all scenarios are looked into.
Perhaps you should read what people actually say before making comments, you might not look like such an idiot then.
Rocket Pepper said:
smart51 said:
Why do you think top gear is in any way important? It will make no difference to the efficiency at all. All gears are equally lossy
And all engines behave differently which makes your statement ridiculous in the context of 20mph efficiency. There are many different cars and many different engines. Enforcing a blanket 20mph limit stating its the most fuel efficient speed can only be so for certain vehicles. In such a study it's the percentage of those most fuel efficient that would support such criteria - or not, as your below comment appears to suggest also.smart51 said:
but the peak in this example is at 20 MPH in second, Probably because the lombardini engine is fairly low revving, like a diesel
So what happens at 20mph in 2nd gear in a fairly high revving engine? Or how about a fairly low revving long stroke engine at 20mph in 5th gear? Any study needs to make comparison based on real world data. Plucking data from suitable engine\transmission combinations that suit a particular argument\debate\study\whatever is floored. It's that simple and not worthy of debate until all scenarios are looked into.
Just in case you can't be bothered to re-read the thread, this is what I said:
smart51 said:
Whilst designing my current car, I worked out that a typical car is most efficient at around 30 MPH.
I then went on to say:smart51 said:
The curve is pretty wide and any speed between 20 and 40 is good.
How could you possible think I was condoning a 20 MPH speed limit after I had said this?I was more concerned about your generalisation over top gear actually. You appear to contradict yourself was my point. But generalising will make you do that of course. Such will also prove that for all your studying it's pretty obvious the simplest way to save fuel is to drive in the sweet spot where the motor is not being laboured for any given speed you wish to move along at, whatever gear that suits the purpose best.
In other words, it's possible to have the best fuel economy at 20mph in one vehicle, and entirely possible to have equally as best economy in an altogether different vehicle at 50mph. Unless all vehicles are beta tested the same the whole thing is a load of old codswallop. Which will only serve to show that each operator knows how to get the best economy from the vehicle they're operating. So please, tell me something I don't know before you chuck insults at me.
In other words, it's possible to have the best fuel economy at 20mph in one vehicle, and entirely possible to have equally as best economy in an altogether different vehicle at 50mph. Unless all vehicles are beta tested the same the whole thing is a load of old codswallop. Which will only serve to show that each operator knows how to get the best economy from the vehicle they're operating. So please, tell me something I don't know before you chuck insults at me.
I'd like to see a bit of research into the environmental impact of speed-bumps, chicanes and other "traffic calming" obstacles.
I'm sure my car would be more efficient travelling at a constant speed rather than the repeated accelerating and braking my local council likes to encourage...
I'm sure my car would be more efficient travelling at a constant speed rather than the repeated accelerating and braking my local council likes to encourage...
matt0677 said:
I'd like to see a bit of research into the environmental impact of speed-bumps, chicanes and other "traffic calming" obstacles.
I'm sure my car would be more efficient travelling at a constant speed rather than the repeated accelerating and braking my local council likes to encourage...
Amen brother!I'm sure my car would be more efficient travelling at a constant speed rather than the repeated accelerating and braking my local council likes to encourage...
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