Motorway speed limit could be raised to 80mph
- Thread starter Pyscho_Si
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![[TW]Fox](/data/man-of-honor/2009.png){kind=avatar}
There might not be a massive difference depending on car, but there will always be a difference.
The difference in mine is about 10% - so 3-4mpg. Massive? Probably not really. A difference? Of course.
Oh...my...god.
You've really outdone even your own poor standards this time Stockhausen.
Dearie me, I honestly can't facepalm enough to convey my opinion of that statement. Just... wow.
Drag increases in proportionality to the square of speed, but the power output must increase in proportion to the cube of the speed. So doubling the speed requires an 8-fold increase in power (2^3).
It is understandable that around 50mph in top gear produces the best economy as the engine will be operating at a speed that is at or close to its peak volumetric efficiency. If the car is accelerated further, not only is a probable that the point of peak efficiency is passed, but aerodynamic drag will increase with the square of speed not to mention friction from the tyres and any mechanical power/drivetrin component that moves.
I therefore find it highly improbable that any car would produce a return in fuel economy at 80mph that is equivalent to the cars' economy at 70mph.
Incidentally a jet engine gets more efficient with speed, but unless you have a Trent 900 strapped onto the roof on your runabout...
It is understandable that around 50mph in top gear produces the best economy as the engine will be operating at a speed that is at or close to its peak volumetric efficiency. If the car is accelerated further, not only is a probable that the point of peak efficiency is passed, but aerodynamic drag will increase with the square of speed not to mention friction from the tyres and any mechanical power/drivetrin component that moves.
I therefore find it highly improbable that any car would produce a return in fuel economy at 80mph that is equivalent to the cars' economy at 70mph.
Incidentally a jet engine gets more efficient with speed, but unless you have a Trent 900 strapped onto the roof on your runabout...
At 80MPH my CTR is doing 4000RPM!! If I reset the trip computer and drive for a few miles it shows around 30/31MPG
At 70MPH it's doing around 3500RPM and gets around 35/36 MPG!!
However, this is obviously just a rough guideline for myself as there are many factors that can vary the results
At 70MPH it's doing around 3500RPM and gets around 35/36 MPG!!
However, this is obviously just a rough guideline for myself as there are many factors that can vary the results
Associate
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What about between 70 and 100mph or even between 70 and 120mph, since the faster you drive, the more efficient a car appears to become in your alternative reality
Of course, that isn't what I said or even implied is it? I said a car will become more efficient up to a point - and where that 'sweet spot' is will depend on the car. It isn't impossible that certain cars designed and geared for motorway cruising will have maximum efficiency higher than 50mph. I do, however, think it's unlikely that a car would consume less at 80 than 70, but it's perfectly reasonable that the difference would only be a few mpg.
All this is irrelevent of course, as if you wish to go at 50mph on the motorway to save money, even with an 80mph speed limit -shock horror- you will still be able to. It would just decriminalise half the drivers on the motorway who already do that speed.
Jawohl, mein Fuhrer !
A lot of things aren't legal because people are misguided or authoritarian arses...
But, to give a couple of examples: Driving abroad, track days, private roads, emergency's...
You honestly think the police solving more theft, violence, fraud, etc is less important than them wasting time on hard working people doing a harmless infraction ?
I also don't think there are many cars where you will find 66.6-100 mph/1000rpm in any gear (as that's what you are implying by saying 80 mph is only 100/150rpm more than 70 mph).
[TW]Fox;18619832 said:
It isn't the 'Eco box' version. When I drove from here to Yorkshire it done 42.7mpg at 70mph. The trip home I went at 75/80 for some of the way and got 40mpg. At that sort of revs the turbo is constantly spinning/on boost to provide maximum power, thats why the mpg isn't great. Sat at roughly 50mph I get 50mpg.
I realised after my post that extra urban is a mixture, even then on roads like that i dont know of a single owner of my car that achieves anywhere near what Fiat claim. I think its a farce.
Just to go back to the cars over 70mph argument, I ride a 125cc 13bhp motorbike which I have had an indicated 83 out of it, and that wasn't easy or quick to get to, and drive a 300bhp hatchback, and on the bike, it is very scary the lack of power that it has, I have no power on tap to get out of tricky situations whereas in the car, I have power whenever I need it, and is a much easier and a safer drive. Can't wait to get a bigger bike. So limiting cars or dogging it's performance would be a silly move in my eyes.
Soldato
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[TW]Fox;18617118 said:
True however it's not a linear increase as you think it is, by your definition 30MPH in top gear at idle RPM is the most efficient speed for a car.
It's a combination basically of aerodynamics, rolling resistance and engine efficiency per RPM, out of the three listed only rolling resistance could be said to be linear (aerodynamics get's complex very quickly :/).
However generally for cars the drag increase from small increases in speed offsets anything else and thus in 99% of cases above 50-60MPH the MPG does in fact start dropping.
As for a turbofan engine getting more efficient with speed, no, I'm afraid it doesn't exactly work like that.
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One thing that affects my mpg more than anything is the air temperature. December with its -5C frosts was costing me at least 5-7mpg against average 15C type temperatures. Maybe this is an issue with diesel.
One other aspect on the main topic which I seem to have caused some arguments is in modern lean burn diesel engines the mixture will be controlled by some sort of engine management chip. It is not inconceivable that at stages the mixture will be richer and at others it will lean out again.
I am now going to conduct some trials over the next few trips which will may take a while and hopefully prove my point. Top up, drive 100 motorway miles plus at a set speed, top up again, a percentage point or two, possibly, 10-15% never.
This may be one difference between a slow revving diesel and a higher revving petrol engine.
And yes it is about 250-300rpm for 10mph difference in 6th gear.
One other aspect on the main topic which I seem to have caused some arguments is in modern lean burn diesel engines the mixture will be controlled by some sort of engine management chip. It is not inconceivable that at stages the mixture will be richer and at others it will lean out again.
I am now going to conduct some trials over the next few trips which will may take a while and hopefully prove my point. Top up, drive 100 motorway miles plus at a set speed, top up again, a percentage point or two, possibly, 10-15% never.
This may be one difference between a slow revving diesel and a higher revving petrol engine.
And yes it is about 250-300rpm for 10mph difference in 6th gear.
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To save you hunting around for it, here is what I actually wrote:
Aside from that strawman, do you actually have any sensible point at all to make?
Soldato
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colder temperatures mean denser air, which has two effects. Firstly there will be a slight increase in power as more air is available at any one time for the engine. Secondly, if the air is denser (thicker) it is more difficult to punch a hole in it.
Can he be barred from posting in the Motors forum please?
He should have been banned full stop.
He's been caught on at least one occasion recently lying just to wind people up.
He's a troll. I'm surprised some people even bother to respond to his deliberate attempts to wind people up with comments like this :
I know exactly what you wrote, you wrote you doubt it's any waste of time with them doing what they do, so obviously, you think it's more important than other crimes, which they often don't have time for because they're wasting their time on nonsense like this.
I'm saying 90% of traffic cops should immediately stop wasting their time on traffic and do more important things.
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You may know what I wrote, you may even think that you understood it. However you don't - read it again.
As a clue, see if you can find many reported incidents of the Police "fining people for doing 32mph in 30mph zone" - that should keep you busy for a while
Perhaps Cameron will share your point of view and make 90% of traffic cops redundant. I still very much doubt that he will allow an increase to 80mph on motorways, even for a fistful of votes.
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Motorway speed cut to save fuel
They're comin' to getcha next!