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Author Topic: MPG.V. speed  (Read 2941 times)

portside II

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MPG.V. speed
« on: May 24, 2008, 12:17:24 pm »

Whilst driving to Hull the other eve ,the wife put a question to me .
My little car does 45mpg at a steady 60mph ,if i go at 70mph the fuel comsumption goes down to 42mpg .
Now the difference in speed (on the motorway) is 10 mph and the drop in mpg is 3 mpg , now my brain has stopped so is it more advisable to travell at 60 or 70 mph for a better mpg ?.
daz
ps. does some one have a chart so it makes working out easier.
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BobF

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Re: MPG.V. speed
« Reply #1 on: May 24, 2008, 12:26:48 pm »

Hi Daz,
Capt.MPG here.
It will take you longer, but 60mph will give better mpg.

Bob
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catengineman

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Re: MPG.V. speed
« Reply #2 on: May 24, 2008, 04:10:55 pm »

55 will be even a tad better but the journey time will be longer
Still more time to look around and shorter stopping distance's time/distance/reaction = less accidents (if only everyone?) O0

R, just IMHO
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portside II

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Re: MPG.V. speed
« Reply #3 on: May 24, 2008, 05:22:58 pm »

Thanks lads , this was giving me a headache ,first thoughts was that time spent on the road would be shorter and the mpg would be better but like you said Bob it is better the other way round  .
So the result in top gear fashion No to clarkson and the Hamster ,Captain slow wins .
daz
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catengineman

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Re: MPG.V. speed
« Reply #4 on: May 24, 2008, 06:22:44 pm »

Being a sort of engineer type chap I can tell you that if you can it is a good idea to run your engine at its optimum speeds.
Working on ships engines and lots and lots of calculations I can tell you that a pair of
EWSL 16 Lister Blackstone's run at their optimum at 955 rpm with the intercooler at 68deg with a diff of 12deg. as against the rated running speed of 1125 rpm.  The difference in fuel consumption was 4.21 tonne at the 955 rpm and 6.05 tonne at 1125 rated  rpm (total waste of typing! for cars you can use the same principle)
I did it with my car through the urban cycles.  Same route, different speeds. with a measured gallon of fuel and setting the trip at the start of each run. (by the way this was when fuel was pence per gallon) I found that the car ran furthest at an average speed of 55 Mph. As I have said this was a few year ago and the techi bits in auto engineering have come a long way but cars are heavier so I think one will count out the other in the swings and roundabouts theory though I drive at the 55 / 60 mph regeon and find that my fuel consumption is steady, if I touch the 65 /70 mph I notice the extra fuel I pay for at fill up time

R,
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JayDee

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Re: MPG.V. speed
« Reply #5 on: May 24, 2008, 07:42:18 pm »

Hello,
The gearing on my Estate car is rather  high.
Travelling on the motorway at a steady 60 mph, uses MORE petrol than travelling on the same road at 75 mph.
This is because the engine is working better at the higher speed, it is "on Cam " at 75 and off cam at 60.

 It all depends where max Torque and Power happen on a particular engine.

Racing engines have max power and torque at very high revs, Touring cars have their power and torque lower down to give nice easy motoring.
A car set up for touring will enable the driver to travel great distances without having to change gear.

Problem is, 75mph is a bit too fast !!.

John, O0   O0  O0
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tobyker

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Re: MPG.V. speed
« Reply #6 on: May 24, 2008, 11:56:10 pm »

For heaven's sake, anyone knows that you always go as fast as possible in order to get home before the fuel runs out!
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tigertiger

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Re: MPG.V. speed
« Reply #7 on: May 25, 2008, 03:54:57 am »

And then there are vehicles with varying cams, varying timing, or variable exhaust valves to tune the pipe at different revs (only know of bikes that do this one).

but in answer to the original post,

Don't mix up MPG and MPH with GPH  ;)
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Peter Fitness

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Re: MPG.V. speed
« Reply #8 on: May 25, 2008, 10:17:37 am »

If cars use more fuel the faster they are driven, then a lot of people here in Britain must have high fuel bills. As an Aussie who has just spent 5 weeks driving around this beautiful country, I am astounded how fast many drivers go on any class of road, whether it be motorways, A or B roads. I know I'm getting old, but I've never considered myself a slow driver, I will travel at the speed limit where possible, and in most cases, if anyone passes me they are exceeding the limit. Over here, however, I always felt as if I was going to be late for something, as most others passed me at speeds well above the limit, or if they couldn't pass, tailgated me. Even in the Lake District, where the roads are narrow and winding, the speed of some drivers was unbelievable. In some parts of England, tailgating seems to be a national sport.

Having said all that, I find most British drivers to be very courteous in other ways, such as allowing others to turn across a line of oncoming traffic in towns, merging on motorways, and stopping on very narrow roads through villages to allow an approaching vehicle to pass a parked car. I did find the British habit of parking facing the wrong direction to be rather off-putting, as this behaviour is highly illegal in Australia, and attracts a heavy fine.
Peter.
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sheerline

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Re: MPG.V. speed
« Reply #9 on: May 25, 2008, 11:05:04 am »

Catengineman has it right, run your engine at its optimum speed, this means selecting the correct gear for the road speed of the vehicle. My own car appears to be most efficient around the 2000 -2500rpm mark, it is wrong to assume that a higher gear and lower rpm will automatically give better fuel consumption. Then there is the drag factor of the vehicle, above 50 mph this increases qiute dramaticatlly ( I don't have the formula so can't quote it), and shape of a vehicles bodyshell which will drastically contribute to high fuel consuption. I am amazed at how many people drive around in square boxes with wheels on each corner and push them through the sound barrier and having driven vans in the past, I know how bad this can be. Although weight of vehicles will contribute to fuel economy, it is amazing how small the effect can be if the vehicle is driven lightly and the engine left unstressed by careful use of gearing
With modern 'efficient' engines, lightweight aerodynamic bodyshells and smooth straight roads, we should be able to really squeeze some fantastic fuel economy figures out of a modern vehicle but unfortunately everyone has his own choice of vehicle styling and method of driving and since, as Peter says, everyone is belting it up hill and down dale, we will never see it.
Has anyone noticed how the words 'drag co-efficient' have dropped completely out of the vehicle manufacturers vocabualry over the years and it is no longer used as a selling point .....this is one of the most vital bits of information one need when soourcing a vehicle with economy and sped in mind!
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malcolmfrary

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Re: MPG.V. speed
« Reply #10 on: May 30, 2008, 12:50:11 pm »

Actually on a run, yes, running the motor at its optimum rpm will give benefits, but when doing a lot of stopping and starting, gentle accelleration pays huge dividends in fuel economy.  Many years ago I had a Viscount, 3.3 and automatic and HEAVY - on a run, including towing a mates car, 25mpg.  Driven with youthful exuberance (it was a long time ago), single figures.
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dreadnought72

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Re: MPG.V. speed
« Reply #11 on: May 30, 2008, 02:44:26 pm »

Then there is the drag factor of the vehicle, above 50 mph this increases qiute dramaticatlly ( I don't have the formula so can't quote it)
The quick rule of thumb is that aerodynamic drag is proportional to the square of the velocity.

So a car at 80mph (the de facto UK motorway speed) is 2.1 times "draggier" than at its (typically) most efficient 55mph.

Andy
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boatmadman

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Re: MPG.V. speed
« Reply #12 on: May 30, 2008, 05:39:18 pm »

Tyre pressures also influence mpg, underinflated tyres need more energy to turn them.

Air con adds approx 10% to the fuel consumption.

Driving with your windows open will increase your mpg by up to 25% - depending on speed. This is because opening windows destroys all the carefull design work to get smooth airflow around the body shell.

Ian
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Bryan Young

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Re: MPG.V. speed
« Reply #13 on: May 30, 2008, 07:11:48 pm »

If cars use more fuel the faster they are driven, then a lot of people here in Britain must have high fuel bills. As an Aussie who has just spent 5 weeks driving around this beautiful country, I am astounded how fast many drivers go on any class of road, whether it be motorways, A or B roads. I know I'm getting old, but I've never considered myself a slow driver, I will travel at the speed limit where possible, and in most cases, if anyone passes me they are exceeding the limit. Over here, however, I always felt as if I was going to be late for something, as most others passed me at speeds well above the limit, or if they couldn't pass, tailgated me. Even in the Lake District, where the roads are narrow and winding, the speed of some drivers was unbelievable. In some parts of England, tailgating seems to be a national sport.

Having said all that, I find most British drivers to be very courteous in other ways, such as allowing others to turn across a line of oncoming traffic in towns, merging on motorways, and stopping on very narrow roads through villages to allow an approaching vehicle to pass a parked car. I did find the British habit of parking facing the wrong direction to be rather off-putting, as this behaviour is highly illegal in Australia, and attracts a heavy fine.
Peter.
During my last visit to Aussie an Adelaide cop let me drive his 4 litre "thing" for a day (he was with me). Went like a bat out of hell, and when I wanted to slow down, his only reaction was "Who is going to stop a cop?". A great day out.
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portside II

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Re: MPG.V. speed
« Reply #14 on: May 31, 2008, 12:04:42 am »

Well for the last week or so i have (to the aproval of the wife) been driving more carefully (slower)  :angel: and when it came to putting some petrol in i found that where i would have put 20. for 160 miles i could only get 18. in the tank .
Not sure how accurate the mpg trip thing is on the carbut i am getting 48-51 mpg where as before it was around the 42-45mpg .
Happy daz.
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malcolmfrary

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Re: MPG.V. speed
« Reply #15 on: May 31, 2008, 11:06:28 am »

Quote
Went like a bat out of hell, and when I wanted to slow down, his only reaction was "Who is going to stop a cop?".
Then again, he wasn't paying for the juice or the tyres and he had somebody else to keep it in good repair.
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tigertiger

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Re: MPG.V. speed
« Reply #16 on: May 31, 2008, 11:37:26 am »


Driving with your windows open will increase your mpg by up to 25% - depending on speed.

Or maybe decrease it  ???

Also a blowing exhaust can reduce MPG by 5%.
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boatmadman

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Re: MPG.V. speed
« Reply #17 on: May 31, 2008, 12:03:13 pm »


Driving with your windows open will increase your mpg by up to 25% - depending on speed.

Or maybe decrease it  ???

Also a blowing exhaust can reduce MPG by 5%.
DOH! you are right as ever TT ;D
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Ghost in the shell

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Re: MPG.V. speed
« Reply #18 on: June 03, 2008, 04:39:47 pm »

well travelling 20 miles ro work and 20 miles back from work on the M6, My car uses per day 1/4 of a tank! thats about 15 to 20 mpg, only run her at 70 on the motorway.

travelling to wicksteed, I did the return trip on half a tabk and used the backroads, never going avove 50
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GaryM

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Re: MPG.V. speed
« Reply #19 on: June 03, 2008, 04:55:49 pm »

Hi
According to the Department for Transport driving at 70mph uses up to 9% more fuel than at 60mph and up to 15% more than at 50mph. Cruising at 80mph can use up to 25% more fuel than at 70mph.

To reduce fuel consumption keep windows closed, remove unused roof racks, accelerate and decelerate smoothly and change up asap.
Source AA
Years ago the Department for Transport recommended 56mph as the ideal for fuel economy.

regards
Gary
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Mankster

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Re: MPG.V. speed
« Reply #20 on: June 03, 2008, 07:12:02 pm »

Fuel used is dependent on how much you depress the throttle pedal (more air to the engine , means more fuel is injected into the cylinderes), so when going down hill you can idle and do silly speeds and not use much fuel, but if you slowed down your MPG may actually go up as it will take longer for you to cover the same distance  :)

GaryM

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Re: MPG.V. speed
« Reply #21 on: June 03, 2008, 10:19:59 pm »

I agree it may save fuel, but bear in mind - I think it's illeagle, your not in "full control of the vehicle" or something like that. :police:
Done it a few times myself when praying to the god of petroleum and the car - please get me home! ;D

regards
Gary :)
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Ghost in the shell

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Re: MPG.V. speed
« Reply #22 on: June 03, 2008, 10:32:47 pm »

aparantly being 10 feet behind a big rig can save you fuel as well, though its not recomended
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cos918

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Re: MPG.V. speed
« Reply #23 on: June 03, 2008, 11:47:05 pm »

hi one other factor to think about is getting the car ecu (brain) re mapped or chipped. When a car maker brings out a car they have to cover all driver from the little old Granny to the boy racer . So the set the fulling a little high , this means there is no danger of the air/fuel mix leaning out and melting engines . This would be bad news on a new car as the car maker would have to cover it under wantry. Supper chips re map the ecu still keeping safety margins but you see better throttle responce and better MPG.
  Also one to concider is tyre brand. If you go for a soft compound tyre you get better grip worse wear and only slightly worse mpg. If you go for a hard compound tyre you get worse grip better wear and only slight better MPG

john
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