Is there any ballpark way of working out what the most economical crusing speed for any given car is, based on things like speed, engine revs, presumably torque/power characteristics?
Im thinking they cant all be the same and I did wonder if say my 2.0 Daewoo will be more economical in hilly areas than my Carina due to better torque - going up a hill out of town, the Carina drops to 3rd gear and is doing about 2700 rpm whereas the Daewoo tugs up in 5th with light throttle at around 2100 rpm.
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I'd be interested in knowing if there is a formula to do this as well.
It the moment, I just look at the instant readout of MPG which is one of the four dials provided on the car.
Somewhat frustratingly, when driving on the level, it seems to show best fuel consumption at either 60mph, or just over 80mph. I find the one a bit too slow and the other too fast.
However at 70 mph, I suffer a drop of about 5 mpg.
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It is lying to you, Londoner.
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ISTR reading somewhere that above 56mph air resistance becomes the overriding factor in increasing the amount of energy required to maintain vehicle speed (something to do with the atomic structure of air, and shear forces, or something).
I believe that's one of the many reasons why certain classes of larger vehicle are limited to around this speed, and also why economy figures were taken at this speed in years gone by.
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There isn't one - it's a mythical idea.
As you go faster, more power is required to overcome drag, with aerodynamic drag power in particular becoming proportional to the cube of speed.
For most people, their time is far more valuable than the fuel they're burning.
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Oy've driven 4 speed auto's for many a year so using my Vulcan logic the car is running efficiently (re: time/motion) when 4th gear comes in, which is around 50mph.
In hilly areas (like down ere) I always 'take a run' at the hill and try to 'sail' up :)
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>> Oy've driven 4 speed auto's for many a year so using my Vulcan logic the
>> car is running efficiently (re: time/motion) when 4th gear comes in, which is around 50mph.
Lots of auto boxesd are linked to km/h speed. my opel kicks into 4th at 80km/h/2000rpm, and therefore at 120 is running at 3k revs.
Cruise all day at that speed, and record silly figures on the fuel economy.
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Faster you go, more fuel needs to be burnt to overcome air resistance.
50 MPH is quite high speed for 4th gear to kick in! In my Ceed auto (4 speed), the 4th kicks in around 35 MPH (when driving on level ground).
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>> 120 is running at 3k revs<<
A fast idle then :)
I say my 4th gear comes in around 50mph, but yes its got to be less than that me thinks,
I had an old model BMW 525e a few years back (nice car) and that was about say 2.5 ltr IIRC, straight six,
when 4th gear came in, the rev counter used to hit the deck almost at 52mph. e = economy.
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>> I had an old model BMW 525e a few years back (nice car) and that
>> was about say 2.5 ltr IIRC, straight six,
>> when 4th gear came in, the rev counter used to hit the deck almost at
>> 52mph. e = economy.
>>
Not being anal but wasn't it one of the first of the 3 digit beemers where the last two digits did not actually signify the engine size? I thought it was a 2.7 litre (but I stand to be corrected).
Nice car and ahead of its time either way.
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>>I thought it was a 2.7 litre (but I stand to be corrected).<<
You're not wrong a900ss ~ www.carandclassic.co.uk/car/C165407/
mine was Silver too, economical (for the size of engine) and nice 2 drive.
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Dog said: "Oy've driven 4 speed auto's for many a year so using my Vulcan logic the car is running efficiently (re: time/motion) when 4th gear comes in, which is around 50mph."
Cripes. My petrol engine will run nicely in 5'th from 30 mph and give good mpg. I get a noticeable improvement in summer, when the air is thinner. To state the obvious, better mpg can be achieved by not braking, by for example matching speed the cars and hazards ahead.
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>>Cripes. My petrol engine will run nicely in 5'th from 30 mph and give good mpg<<
I updated that post @ 16.53 to say ...
just checked my 1.8 Almera auto, and it will drop into 4th @ 40mph which equates to about 1600 rpm,
If I drive at that speed (mimse?) I can get 45 mpg on the ole puter.
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>>There isn't one - it's a mythical idea.<<
So if there isnt one, a car should give the same economy no matter what gear/revs or speed?
Id always assumed there was an economy curve, like a torque curve, for every car which would vary depending on a number of different factors.
I mean, a 2CV, at 70 mph would presumably be near its rev limit and using a fair amount of fuel, maybe more than a larger car with a less stressed engine, but at slower speeds, the 2CV might be more economical. I guess not!
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as a rule of thumb, and works for any car, the less revs it using at any given speed, the less fuel its using.
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>>So if there isnt one, a car should give the same economy no matter what gear/revs or speed?
No, that doesn't follow.
What I'm saying is that as the power required to move the car increases rapidly with speed, going faster will mean that more of the energy in the fuel gets wasted moving air rather than moving the car forwards, and the most economical speed is also the slowest.
The power required to shift the air being a function of speed cubed means that it dominates most other considerations - especially at higher speeds, and is the reason why the updated Veyron needed lots of extra horsepower to provide a small increase in its top speed.
There may be local oddities in the engine's response map where you can get slightly better mpg than you might expect, but the general trend is as I've described.
>>Id always assumed there was an economy curve
For an engine, there is, it's a performance map - I've posted links to them in the past. Broadly, you get the best *specific* fuel consumption at mid load, and about the engine speed for maximum torque.
By specific consumption, I mean fuel used per unit power. As above, the power going up with speed cubed means that the small changes in the performance map are usually insignificant.
People massivley over-estimate the effect that engine revs have on fuel consumption. The power required to move a car at a given speed is fixed.
>>as a rule of thumb, and works for any car, the less revs it using at any given speed, the less fuel its using.
It's possible to take this too far, but generally it's correct. If you end up with the engine turning over much slower than the speed for maximum torque, you've gone too far, i.e., you don't actually want to end up with the car just above tick over speed - it actually wouldn't be as efficient as at the speed for maximum torque.
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Thanks chap, that was the sort of comprehensive answer in pleb language I was hoping for :-)
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>> For most people, their time is far more valuable than the fuel they're burning.
And, it seems, the load space and how high they are sitting. Seems to me that nowadays, aerodynamics has taken a back seat to trendy, boxy designs that increase fuel consumption. Hence the increase in small vans, people carriers and crossover cars like the new Nissan Juke. Or maybe the public isn't to blame and they're just having to buy whatever the manufacturers decide the public want to drive :-)
Last edited by: corax on Fri 13 Aug 10 at 22:05
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I have found that maintaining a nice 45mph on NSL A-roads gives good economy.
Those following me must do even better, as they often appear close enough to be slipstreaming me....
It also helps to maintain that speed through villages, etc.
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Also helps to keep in the middle of the road
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Yus ... just checked my 1.8 Almera auto, and it will drop into 4th @ 40mph which equates to about 1600 rpm,
If I drive at that speed (mimse?) I can get 45 mpg on the ole puter.
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"the most economical speed is also the slowest."
Does this mean that if my car is almost out of fuel, and I am desperately hoping to make it to the next petrol station, my best bet is to go along at 10 or 15 miles an hour in 2nd gear or something like that?
"People massivley over-estimate the effect that engine revs have on fuel consumption. The power required to move a car at a given speed is fixed."
Does that mean that at, say 40 mph, it doesn't really make much difference to fuel economy whether I'm in 4th or 5th gear? (Obviously this will vary a little from car to car, depending on gearing and other factors - but as a general rule of thumb, is it true?)
And how much effect do engine revs actually have on fuel consumption in an average car - e.g. a 1.6l petrol Ford Focus?
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70 down the big hill back from scarborough in neutral
watch for the tractor pulling out from that old pile of house on the right though castle haven or something......
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>> Does this mean that if my car is almost out of fuel, and I am
>> desperately hoping to make it to the next petrol station, my best bet is to
>> go along at 10 or 15 miles an hour in 2nd gear or something like
>> that?
At fairly low speeds, you aren't wasting much fuel against aerodynamic drag, so, I would suggest that 30 - 40 mph in a high gear would be a reasonable compromise.
>> Does that mean that at, say 40 mph, it doesn't really make much difference to
>> fuel economy whether I'm in 4th or 5th gear? (Obviously this will vary a little
>> from car to car, depending on gearing and other factors - but as a general
>> rule of thumb, is it true?)
Difficult to give a general answer, but at 40, this might be slowing the engine below its optimum speed. One way to see this is to compare how far you need to open the throttle to make the engine rev at, say, 4000 rpm while it is stationary compared with the much larger pedal travel that you will need to apply to make the engine reach 4000 rpm in a high gear.
>> And how much effect do engine revs actually have on fuel consumption in an average
>> car - e.g. a 1.6l petrol Ford Focus?
All of these questions would be best answered by looking at the performance map for the engine, and the drag power required to drive your car. Oddly, I imagine that manufacturers rarely get asked for these data nowadays.
I'll see if I can find a link to decent engine performance map, hopefully one that has some constant power lines superimposed, and discuss it in some more detail.
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Hopefully, I'll find a better one, but, page 24 of this document shows a performance map;
www.engine-expo.com/forum_2009/pdfs/day1/11_robert_walling.pdf
The black contours are lines of constant *specific* fuel consumption.
The red contours are lines of constant power - these are entirely equivalent to being lines of constant vehicle speed - there's a unique relationship between vehicle speed and power required for any given vehicle (and vehicle trim)
So, to answer the general question about if I'm going N miles per hour, should I be in 3rd, 4th, or 5th, you need to do the following;
- find the "road load" power which corresponds to the speed you're interested in, and find the nearest red contour
- for that speed find the engine rpm which corresponds to the road speed you're interested in
- plot these points onto the identified red contour
- the gear which is associated with the lowest BSFC contour will give you the best economy
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You can see in this particular map, the best BSFC is obtained at quite a high engine load [the "centre of the onion" is high up on the graph, just below the maximum torque line], and between 2500 and 3000 rpm. For this particular engine, it will be virtually always true that highest gear is best. However, some engine have the best BSFC at lower engine loads, and it's possible to pass through the centre of the onion, and out of the other side, back towards lower economy.
The whole of that map doesn't cover a factor of 2 in BSFC, between 240 g/kWhr and ~ 400 g/kWhr whereas the ratio of power required at, say 30 mph and 80 mph is 18.9, i.e., the speed you drive is a much stronger influence on fuel economy than what gear you choose to drive in.
I'm sure that the map is not easy to digest if you haven't considered one before. There are lots of complex factors which go to produce the map, but, the location of the centre of the onion is largely governed by;
- the engine speed where volumetric efficiency is highest
- larger throttle openings produce good BSFC, as the losses due to throttling are less
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My head was somewhere below that. ;>)
For a petrol, I'd say try not to keep it below 2K RPM at large throttle openings, otherwise low revs are good.
I've got a regular journey that takes me through a 40MPH, SPECS camera limit for about ten miles, then another ten miles through another 50MPH, SPECS camera limit onto the M25 roadworks!
The journey times aren't significantly longer - they just seem that way. It does wonders for the fuel economy.
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WhatCar? do this every 5-6 years - take a bunch of cars then analyse MPG at various constant speeds.
Generally all cars are better at 30mph than at each 10mph increment above that, with a couple of exceptions where 40mph is better - presumably due to long gearing vs engine mapping, or lock-ups not being used in Torque converter autos.
edit: I remember seeing an AA test from the late 70s on a 2.0 Cortina at constant speeds in 4th (top) in increments of 5mph from 20mph to 80mph.
Peak mpg was at 25mph - obviously can't be directly compared to todays fuel injected cars with overdrive top gears.
Last edited by: Lygonos on Sat 14 Aug 10 at 00:43
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I have not filled up yet to prove anything but went to Yorkshire last weekend in the Mazda6 diesel. Filled up the day before going.
Done about 320 miles since I filled up and the range still says about 200 left in the tank (so possibly close to 300 really based on how pessimistic the fuel gauge and range is - it can say empty and zero miles and have 7 or more litres still in it!)
Driving back to home I went on a detour so did mostly A roads and some motorway... but those A roads will have made a difference.
EDIT: Detour was deliberate... got some nice new homeware/crockery in Boundary Mill in Colne ;-)
Last edited by: rtj70 on Sat 14 Aug 10 at 00:54
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>> You can see in this particular map, the best BSFC is obtained at quite a
>> high engine load [the "centre of the onion" is high up on the graph, just
>> below the maximum torque line], and between 2500 and 3000 rpm.
As Marvin Gaye used to say, the world is just a great big onion.
;-)
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...the world is just a great big onion...
And hate and fear are the spices that make it fly
And the only way to get rid
Of this great big onion
Is to plant love seeds until it dies
tinyurl.com/3266eed
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Thanks for those posts, Number Cruncher.
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The instruction manual for my Ferguson tractor, 1949, gives a good rule of thumb, which surprisingly, I think applies to a car too:
The test for whether you are overdoing the economy attempt by driving in too high a gear is to flip the throttle open a bit more. If the vehicle accelerates, the gear is not too low. But if it is sluggish and won't accelerate, you are in too high a gear for best economy.
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"My head was somewhere below that. ;>)
For a petrol, I'd say try not to keep it below 2K RPM at large throttle openings, otherwise low revs are good."
And my head likewise. In fact, I'd say that my head is probably a bit below yours since the phrase "large throttle openings" challenges me.
Based on my own experience, by the way, I find that when driving on the level, 1500 rpm in 5th gear is about right on my Berlingo, which has a PSA TU5 petrol engine. Dropping below 1500 fails the Cliff Pope tractor test, and rising above 1500 sees economy deteriorating. (My guess is that if I really wanted good economy, I'd drive at 1500 rpm in 3rd gear. But that would be seriously inconveniencing to other road users.)
However, on my Ford Ka, with a 1.3 Duratec engine, it really requires 2000 rpm or above to cruise comfortably.
Last edited by: tyro on Sat 14 Aug 10 at 15:59
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>>"large throttle openings" challenges me.
By the sounds of it, actually, it doesn't.
If you're running your car between 1500 and 2000 rpm, then you'll almost certainly be running at a larger throttle opening than you would be if using a lower gear.
As you move along the red constant power lines, towards the low rpm end of the scale, the red line rises [as it must], this means that the engine is running at higher load, and hence the throttle will be further open.
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I drove down from Manchester to Luton Airport this morning and then to north London. I was cruising at an indicated 80mph (cruise control) and I managed a respectable 40mpg whilst driving at this speed, but when I got to a contra flow and reduced to 50mph the economy improved significantly.
What was interesting was that the combined total of miles driven on the tank plus the range remaining was steady at about 550miles, until I got near to Luton with a seven mile contraflow. By the end of the 50-mph zone the combined figure was over 700 miles. Makes you think how much fuel you can save by driving slowly and smoothly. Normally, during the business day with heavy traffic it is impossible, but on a Sunday with little traffic and time to spare, you can really just sip at the fuel.
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>> Normally, during the business day with heavy traffic it is impossible
Normally, during the business day with heavy traffic it's easier! Don't join the lemmings in lane 3 doing 80-60-80-60-80, stick to 65 or so in lanes 1 and 2. Much less stress, no longer journey times. I do this and I see 40+mpg from my clapped-out old 1.8 petrol Escort, it drops to 34mpg if I hammer along with the Audis and I arrive 5 minutes earlier and much more stressed.
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>>Don't join the lemmings in lane 3 doing 80-60-80-60-80, stick to 65 or so in lanes 1 and 2.<<
And lane 1 is generally the quicker lane on the M25 over a significant distance, during busy periods.
Just watch where the private hire and taxis all go!
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A point which I don't think anyone has made is that on a modern auto, the torque converter will almost certainly lock up at a certain speed, on my car (Saab 9-5) around 55mph. Anywhere below that and the engine is turning more than it needs to push the car along. This probably explains why my car is more economical at 70mph than 50mph - but surprisingly, I have found from expperience that there is little difference in fuel economy between an indicated 70mph and 85mph.
More to the point, most cars have a "happy speed" - the speed at which they settle into almost subconsciously. It used to be around 60mph. I suspect, but can't prove, that this is where the engine is turning over at the rpm corresponding to the peak torque figure, and that it *may* be the most economical cruising speed.
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yes, the "happy speed" is a good way of putting it. On recent long journey I found my A3 2.0 TDi dsg sat happily at 66mph doing, coincidentally, 66mpg (computer readout on the instant mpg setting). Engine was working very leisurely, at (IIRC) at less than 2500 revs. On everyday shorter journeys the car does 49mpg.
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Just found this:
tinyurl.com/362uegf
"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."
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Most economical cruising speed - Londoner
