How much does speed affect the MPG of economical cars
Discussion
As per title, how much does speed change the MPG of cars which are supposedly economical?
My 5.0 v8 does around the same MPG at 90 as it does at 50, basically because the engine doesn't care if you want to do 90 because it can handle it.
In a hypothetical universe (or Germany) where you can do 90mph all day would that good fuel economy in an economical car only be available at low speed and become negligible at higher speed if you have to run it at 4000rpm to cover the same distance in the same time?
My 5.0 v8 does around the same MPG at 90 as it does at 50, basically because the engine doesn't care if you want to do 90 because it can handle it.
In a hypothetical universe (or Germany) where you can do 90mph all day would that good fuel economy in an economical car only be available at low speed and become negligible at higher speed if you have to run it at 4000rpm to cover the same distance in the same time?
I've never owned an V8 but have never driven a car where the economy was the same at motorway speeds as a constant 50mph.
The physics doesn't seem to suggest this to be the case. It was some time ago since I did the maths at uni but fuel consumption should increase, the faster you go.
The physics doesn't seem to suggest this to be the case. It was some time ago since I did the maths at uni but fuel consumption should increase, the faster you go.
ingenieur said:
As per title, how much does speed change the MPG of cars which are supposedly economical?
My 5.0 v8 does around the same MPG at 90 as it does at 50, basically because the engine doesn't care if you want to do 90 because it can handle it.
In a hypothetical universe (or Germany) where you can do 90mph all day would that good fuel economy in an economical car only be available at low speed and become negligible at higher speed if you have to run it at 4000rpm to cover the same distance in the same time?
Which magic V8 do you have? As mine certainly uses more fuel at 90 than at 65/70My 5.0 v8 does around the same MPG at 90 as it does at 50, basically because the engine doesn't care if you want to do 90 because it can handle it.
In a hypothetical universe (or Germany) where you can do 90mph all day would that good fuel economy in an economical car only be available at low speed and become negligible at higher speed if you have to run it at 4000rpm to cover the same distance in the same time?
The only reason your car might be as efficient at 50 as 90 - which I don't believe for a second - is if it's still in a lower gear at 50. Basically, the most efficient speed for a car is the lowest speed it can smoothly drive at in its highest gear. After gearing, the increasing drag at higher speeds determines that economy drops as speed rises.
Your hypothetical question is confusing me, but if you're asking if it's possible for different cars to be the most economical at different speeds, then yes. Fwiw, there's no speed at which your 5 litre V8 is the most efficient.
Your hypothetical question is confusing me, but if you're asking if it's possible for different cars to be the most economical at different speeds, then yes. Fwiw, there's no speed at which your 5 litre V8 is the most efficient.
Just to be clear... I would say it is substantially true that the speed does not affect fuel economy significantly. So put away your protractors and oscilloscopes.
Potentially it is also down to the fact that a car with a larger engine uses more fuel to go slowly (i.e. wasting fuel) so when it is going faster it becomes more efficient for almost the same fuel usage.
I suppose you could work this out if you had access to fuel maps from various engines and you had all the figures to be able to translate what the fuel tables mean in real numbers.
Potentially it is also down to the fact that a car with a larger engine uses more fuel to go slowly (i.e. wasting fuel) so when it is going faster it becomes more efficient for almost the same fuel usage.
I suppose you could work this out if you had access to fuel maps from various engines and you had all the figures to be able to translate what the fuel tables mean in real numbers.
E63eeeeee... said:
Your hypothetical question is confusing me, but if you're asking if it's possible for different cars to be the most economical at different speeds, then yes. Fwiw, there's no speed at which your 5 litre V8 is the most efficient.
I dunno... there are ways of making power which are more or less fuel efficient. When you start turbocharging for instance. Turbocharged engines are economical off boost but use more fuel on boost. Sometimes additional fuel is used as safety against knock and sometimes it can be used to keep the engine cooler. So a naturally aspirated engine making the same power might be more fuel efficient. ingenieur said:
Just to be clear... I would say it is substantially true that the speed does not affect fuel economy significantly. So put away your protractors and oscilloscopes.
Potentially it is also down to the fact that a car with a larger engine uses more fuel to go slowly (i.e. wasting fuel) so when it is going faster it becomes more efficient for almost the same fuel usage.
I suppose you could work this out if you had access to fuel maps from various engines and you had all the figures to be able to translate what the fuel tables mean in real numbers.
Even with the addition of the wibbly "substantially true" and "significantly" you're still wrong. Potentially it is also down to the fact that a car with a larger engine uses more fuel to go slowly (i.e. wasting fuel) so when it is going faster it becomes more efficient for almost the same fuel usage.
I suppose you could work this out if you had access to fuel maps from various engines and you had all the figures to be able to translate what the fuel tables mean in real numbers.
It's an exponential increase, so a small increase in speed from the optimal speed has a small increase in consumption, but the rate that consumption increases accelerates as you increase speed.
There's plenty of information about this if you google it.
ingenieur said:
Just to be clear... I would say it is substantially true that the speed does not affect fuel economy significantly. So put away your protractors and oscilloscopes.
I don’t think you understand physics. What may be misleading is that the internal combustion engine is tremendously inefficient, thus the fuel bunt may not vary as much with speed as one might expect.
ingenieur said:
E63eeeeee... said:
Your hypothetical question is confusing me, but if you're asking if it's possible for different cars to be the most economical at different speeds, then yes. Fwiw, there's no speed at which your 5 litre V8 is the most efficient.
I dunno... there are ways of making power which are more or less fuel efficient. When you start turbocharging for instance. Turbocharged engines are economical off boost but use more fuel on boost. Sometimes additional fuel is used as safety against knock and sometimes it can be used to keep the engine cooler. So a naturally aspirated engine making the same power might be more fuel efficient. At a first approximation fuel use depends on how much power you require. Comparing two cars with the same frontal area and basically the same aerodynamics and weight, power required and therefore fuel used is proportional to speed.
So ignoring engine efficiency, IE looking at the automotive equivalent of a spherical cow in a vacuum, it makes no difference whether you have a 5 litre 8 or a 1 litre 4.
Then efficiency comes into it. At realistic cruising speeds the 5 litre has a far smaller throttle opening to accept that amount of fuel than the 1 litre does, so there are considerable throttling losses caused by sucking fuel/air through a small opening.
On the other hand the 5 litre will be revving more slowly so less friction loss, and the cylinders will be larger so lose less heat.
The designer has to make a decision whether they want to concentrate on efficiency at low revs to make the steady speed fuel consumption figures look good, or to prioritise efficiency at high revs to make the maximum power/speed/acceleration figures look good. Given probable buyers they will probably design the 1 litre optimising low revs and the 5 litre with more emphasis on high rev efficiency. But you might find an anomalous situation in either car when ging a bit faster puts the engine into a more efficient part of the rev range and puts what would otherwise be wasted power to good use.
So at a steady speed the smaller less powerful engine will tend to be more efficient, but the difference is not always that great. If you're comparing basically the same car but with different sized cylinders it may well be the other way round.
In traffic on the other hand, keeping the big engine ticking over will take much more fuel, and in practice the big engined car is probably heavier so takes more power to accelerate.
Trying to compare fuel consumption at different speeds is difficult in practice because the slow speed run invariably involves slowing behind even slower traffic and then accelerating back up when the overtaking lane is clear, while going faster might mean there isn't as much coming up behind and you can move out while maintaining a steady speed.
So ignoring engine efficiency, IE looking at the automotive equivalent of a spherical cow in a vacuum, it makes no difference whether you have a 5 litre 8 or a 1 litre 4.
Then efficiency comes into it. At realistic cruising speeds the 5 litre has a far smaller throttle opening to accept that amount of fuel than the 1 litre does, so there are considerable throttling losses caused by sucking fuel/air through a small opening.
On the other hand the 5 litre will be revving more slowly so less friction loss, and the cylinders will be larger so lose less heat.
The designer has to make a decision whether they want to concentrate on efficiency at low revs to make the steady speed fuel consumption figures look good, or to prioritise efficiency at high revs to make the maximum power/speed/acceleration figures look good. Given probable buyers they will probably design the 1 litre optimising low revs and the 5 litre with more emphasis on high rev efficiency. But you might find an anomalous situation in either car when ging a bit faster puts the engine into a more efficient part of the rev range and puts what would otherwise be wasted power to good use.
So at a steady speed the smaller less powerful engine will tend to be more efficient, but the difference is not always that great. If you're comparing basically the same car but with different sized cylinders it may well be the other way round.
In traffic on the other hand, keeping the big engine ticking over will take much more fuel, and in practice the big engined car is probably heavier so takes more power to accelerate.
Trying to compare fuel consumption at different speeds is difficult in practice because the slow speed run invariably involves slowing behind even slower traffic and then accelerating back up when the overtaking lane is clear, while going faster might mean there isn't as much coming up behind and you can move out while maintaining a steady speed.
I can only assume the OPs fuel economy read out is really imprecise or scientific method is lacking or the engine is reeeaally badly calibrated. All of these are more likely than any size of engine actually requiring less fuel to maintain a higher speed.
I guess in some extreme cases active aero/suspension might make a difference.
I guess in some extreme cases active aero/suspension might make a difference.
If your V8 is pushing the world's slipperiest bobsled-cum-car through the air, you are probably right.
If it pushing a large bluff fronted box on wheels, you are probably talking b
ks.
Energy consumed per mile by aero drag increases with the square of speed, so you will need more than 3 times the energy per mile to go from 50 to 90.
Over 50 mph, drag becomes the dominant consumer of fuel.
So even if nothing else changes with the car's power consumption, you are going to be burning more than 50% more fuel per mile.
The gearing and resulting point on the BMEP map that the engine is operating at between the two speeds may have a small effect on the engine's efficiency, like 5%, possibly 10%.
But it sure as hell won't be more than 50% better.
Unless you are redlining it at 50 and sitting in top gear at 90??
Tell me what the car is and I'll tell you the power required at the driven wheels for the two speeds.
If it pushing a large bluff fronted box on wheels, you are probably talking b
ks. Energy consumed per mile by aero drag increases with the square of speed, so you will need more than 3 times the energy per mile to go from 50 to 90.
Over 50 mph, drag becomes the dominant consumer of fuel.
So even if nothing else changes with the car's power consumption, you are going to be burning more than 50% more fuel per mile.
The gearing and resulting point on the BMEP map that the engine is operating at between the two speeds may have a small effect on the engine's efficiency, like 5%, possibly 10%.
But it sure as hell won't be more than 50% better.
Unless you are redlining it at 50 and sitting in top gear at 90??
Tell me what the car is and I'll tell you the power required at the driven wheels for the two speeds.
I was always led to believe that at any meaningful speed, as speed doubles, frontal drag squares. This is why a 50bhp car will do 100mph but a 100bhp car won't do 200mph.
Obviously as drag increases it takes more power to overcome (Newton's first law - a body will remain at rest or at a constant velocity unless acted upon by an external resultant force), thereby using more fuel.
Physics innit.
Obviously as drag increases it takes more power to overcome (Newton's first law - a body will remain at rest or at a constant velocity unless acted upon by an external resultant force), thereby using more fuel.
Physics innit.
Dr Jekyll said:
At a first approximation fuel use depends on how much power you require. Comparing two cars with the same frontal area and basically the same aerodynamics and weight, power required and therefore fuel used is proportional to speed.
Drag power increases with the cube of speed. But because you spend less time to travel a certain distance, this manifests itself as a square law for the energy per mile consumed by drag.Rolling resistance power increases linearly with speed, so this manifests as a quasi-fixed value for energy per mile.
Small fuel efficiency differences between the two speeds won't come anywhere near making up for the increase in drag losses.
The way you drive inclines and momentum seems to have an effect too.
On many a long and (very) boring trip up and down the (quite hilly) M74 I've played with the cruise control on and off to see how it affected mpg.
With it off I could allow gravity to assist a slight overspeed on a downslope to then allow me to use less throttle on the next upslope with a slow drop back down to cruising speed.
With cruise on (i.e. steady speed driving) I invariably got less mpg.
Like I said...I got quite bored doing those trips
On many a long and (very) boring trip up and down the (quite hilly) M74 I've played with the cruise control on and off to see how it affected mpg.
With it off I could allow gravity to assist a slight overspeed on a downslope to then allow me to use less throttle on the next upslope with a slow drop back down to cruising speed.
With cruise on (i.e. steady speed driving) I invariably got less mpg.
Like I said...I got quite bored doing those trips
Some cars have become very efficient aerodynamically, its true that the slipperiest shapes on the narrowest and least grippiest of tyres won't suffer the steepening fuel usage curve as speed increases, but not many PH'ers would want to drive fast on a set of racing bicycle slicks at 100mph for long.
The worse car we owned for a rapidly worsening fuel curve was a Citroen C2 VTS with the 110HDi Diesel engine, up to around 60mph and general running around it would return 50mpg comfortably, increase cruising speed to 80mph and 35mpg was the likely figure, if you had the same engine in a C5 at 80 then 45+ was still normal, it can only have been that short dumpy shape and the relatively for its half pint size wide low profile tyres causing resistance to rise so sharply.
I'm yet to see the car that returns better fuel consumption at 80 than it does at 50 except maybe in a man-maths calc, whatever the gearing you are shifting masses more air and having to overcome increasing tyre friction, you only have feel the difference if you have a tail wind or benefit from slip streaming to realise the effect wind drag is having.
The worse car we owned for a rapidly worsening fuel curve was a Citroen C2 VTS with the 110HDi Diesel engine, up to around 60mph and general running around it would return 50mpg comfortably, increase cruising speed to 80mph and 35mpg was the likely figure, if you had the same engine in a C5 at 80 then 45+ was still normal, it can only have been that short dumpy shape and the relatively for its half pint size wide low profile tyres causing resistance to rise so sharply.
I'm yet to see the car that returns better fuel consumption at 80 than it does at 50 except maybe in a man-maths calc, whatever the gearing you are shifting masses more air and having to overcome increasing tyre friction, you only have feel the difference if you have a tail wind or benefit from slip streaming to realise the effect wind drag is having.
Edited by Smint on Sunday 5th March 12:58
Gassing Station | General Gassing | Top of Page | What's New | My Stuff