60 MPH Limit on Motorways

[pinballdave]

Wind speed and direction is quite a significant factor at motorway speeds, due to the majority of fuel consumption being due to aerodynamic drag, the difference between a headwind and tailwind can easily be into the double digits of mpg.

So just comparing two trips without taking into account external factors, is not exactly a scientific test.

Besides for the past few decades most manufacturers have optimised their fuel consumption for the official tests at 56mph, so that is usually the most economic cruising speed. If they tuned them so they are more efficient at 80mph, then their 56mph figures would suffer and their marketing department would be having panic attacks.

So once you get into top gear at a speed where the engine isn't labouring, and so you've minimised your mechanical losses, then it's simple physics, any increase in speed will result in a decrease in efficiency due to the drag increasing at a non-linear rate.

Personally I support a reduction in speed limit when the motorway is heavily used. I find it less stressful when all lanes are moving at similar speeds (unless I'm in a hurry!). But on the other hand, if we're going to invest in variable speed limit signs, why can't we raise the speed limit when the roads are nearly empty, the increase in emissions from a tiny number of cars on empty roads will be insignificant compared to the savings from the greater numbers of vehicles during busier times.

[tofufi]

Originally Posted by Laird Scooby

Are you sure about 99%? I believe the x90 has a similar system on the B6304 engine, fairly sure a lot of later Volvos have it, a lot of Hondas, many Vauxhalls have it (Senator 3.0 24v with "Dual Ram") and countless cars out there have VVTi, VTEC or similar which alters the valve timing and/or introduces the second set of valves on each cylinder above certain speeds - all of which achieves a similar result.

That's millions of cars Jim, all with either variable length inlet or variable valve timing and lift that improve the engines efficiency at higher rpm.

With that in mind, it would therefore make sense to increase the speed limit and encourage people to drive faster to get their engines into the optimum range of designed operation.

Also some engines will drop cylinders out completely when cruising above certain speeds and then the economy really shoots up and the emissions drop.

You can see a full list here:

https://en.wikipedia.org/wiki/Variab...ntake_manifold

I've never driven a car which is more economical at 80-90 than 50-60. And I'd be interested to know which common cars have engines which drop cylinders at higher speed (and therefore higher engine load)! All of the ones I know that have cylinder deactivation are to reduce consumption at low engine loads...

Sorry Dave, but the vast majority of vehicles will have better fuel consumption at lower speeds.

Originally Posted by pinballdave

Besides for the past few decades most manufacturers have optimised their fuel consumption for the official tests at 56mph, so that is usually the most economic cruising speed. If they tuned them so they are more efficient at 80mph, then their 56mph figures would suffer and their marketing department would be having panic attacks.

The official tests havn't been done at a the 56mph constant speed for ages.

The NEDC was last changed in 1997, and all new cars now use the WLTP cycle, which has even more changes of speed.

https://en.wikipedia.org/wiki/Worldw...e:Class_3b.svg

[Laird Scooby]

Originally Posted by pinballdave

So just comparing two trips without taking into account external factors, is not exactly a scientific test.

You've missed the point, They were identical journeys in identical weather and traffic conditions, little or no wind and even if there was a headwind on the first day, that would have been a tailwind on the return journey.
Both trips were even underatken at similar times of the evening so unless i did it in a laboratory or somewhere like Millbrook Proving Ground (somewhere Jim knows well) with a track i could just drive on, there isn't much more i could have done to ensure impartiality, that's why the two trips are comparable.

Re your point about 56 and 75mph, i see what you're saying but i was traveling faster and there isn't a test for that so isn't really relevant - there's no "official" standard for those speeds so comparison isn't possible.

You could aargue the point that with 75mph technically being illegal in the UK, why does the govt publish fuel economy figures at 75mph as well................

[Laird Scooby]

Originally Posted by tofufi

You can see a full list here:

https://en.wikipedia.org/wiki/Variab...ntake_manifold

That's nowhere near a full list Jim!

[biggbn]

Originally Posted by Laird Scooby

Are you sure about 99%? I believe the x90 has a similar system on the B6304 engine, fairly sure a lot of later Volvos have it, a lot of Hondas, many Vauxhalls have it (Senator 3.0 24v with "Dual Ram") and countless cars out there have VVTi, VTEC or similar which alters the valve timing and/or introduces the second set of valves on each cylinder above certain speeds - all of which achieves a similar result.

That's millions of cars Jim, all with either variable length inlet or variable valve timing and lift that improve the engines efficiency at higher rpm.

With that in mind, it would therefore make sense to increase the speed limit and encourage people to drive faster to get their engines into the optimum range of designed operation.

Also some engines will drop cylinders out completely when cruising above certain speeds and then the economy really shoots up and the emissions drop.

I'm afraid I strongly disagree with you. The notion a car will be more economical the faster it goes is a non starter for me. My personal experience is at odds with yours, and I can tell you categorically a 3 litre vauxhall likes a drink above 60mph!! I nicknamed my senny 'ollie ' after Oliver Reed!!

[green van man]

Originally Posted by biggbn

I'm afraid I strongly disagree with you. The notion a car will be more economical the faster it goes is a non starter for me. My personal experience is at odds with yours, and I can tell you categorically a 3 litre vauxhall likes a drink above 60mph!! I nicknamed my senny 'ollie ' after Oliver Reed!!

It's a logarithmic progression. Double the speed quadruple the air resistance. The resistance increases by the square of the increase.

This hold true no matter the shape of any car. You will have to overcome 4 times as much air resistance at 60 than you would at 30. This extra work requires extra effort and effort/ work comes from the fuel burned, more work = more fuel.

An engine may have a sweet spot where it runs most comfortably but that is not nessesarily it's most economical.
My D5 is comfortable at either 60 or 80. Put the caravan behind it and it uses a damp sight more fuel at 60 than it would solo at 80, however that is not the point because we come back to work done fuel consumption ratio and always the more work an engine is required to do the more fuel it will use.


Paul.

[Laird Scooby]

Originally Posted by green van man

we come back to work done fuel consumption ratio and always the more work an engine is required to do the more fuel it will use.


Paul.

Agreed but if you can get that engine to operate much more efficiently at slightly higher speed then what i say also holds true.

I know one thing, my 760 that i'm not saying is more economical at 80 than 60 is a right pain at 55-60mph because overdrive keeps dropping in and out - to keep it in it needs to remain consistently above 60mph or i'm driving in 3rd most of the time!

[andy_d]

either way you care to argue it
NO still remains the ONLY sensible choice,
the brain deads will NEVER change there minds , brainwashed they are

if you dont GET the point about YET ANOTHER BS restriction on what we can / cant do being a issue , then you ARE part of the problem
same as all the morons bleating about climate change being due to man
IDIOTS brainwashed by the gov / media to "justify" a whole raft of New taxations.

"give them an inch they take a mile" APPLIES to govment
take the Current BS whereby You HAVE no rights, are being controlled in what/where you can/cant do,, being made to wear Masks *that do NOT one single thing to stop corvid,, take the UTTER BS about the TRACK and trace crap being "to protect you" ,, it can NOT protect you ,, but you dumb brain deads swallow it,, THEN dare to have a go at people who THINK for themselves

the human race NEEDS a wake up call,, "google" doesnt have the answer you NEED to work things out for yourselves AND think for yourselves,, NOT have a go at those who DO and do NOT follow the brain dead heard,

[biggbn]

Originally Posted by andy_d

either way you care to argue it
NO still remains the ONLY sensible choice,
the brain deads will NEVER change there minds , brainwashed they are

if you dont GET the point about YET ANOTHER BS restriction on what we can / cant do being a issue , then you ARE part of the problem
same as all the morons bleating about climate change being due to man
IDIOTS brainwashed by the gov / media to "justify" a whole raft of New taxations.

"give them an inch they take a mile" APPLIES to govment
take the Current BS whereby You HAVE no rights, are being controlled in what/where you can/cant do,, being made to wear Masks *that do NOT one single thing to stop corvid,, take the UTTER BS about the TRACK and trace crap being "to protect you" ,, it can NOT protect you ,, but you dumb brain deads swallow it,, THEN dare to have a go at people who THINK for themselves

the human race NEEDS a wake up call,, "google" doesnt have the answer you NEED to work things out for yourselves AND think for yourselves,, NOT have a go at those who DO and do NOT follow the brain dead heard,

Mmm. What an impassioned outburst. Well done. I will go and have a look, but I don't THINK I'm brain dead, but when a complete stranger who will likely never know me brands me thus I feel compelled to check... I wonder what happened to manners and respect in conversation?

There is a delightful irony in being branded brain dead by someone who suggests we should not follow a 'heard'.... 😂

And yes, I am more than aware that I make spelling and grammar mistakes, but I ain't calling anyone brain dead... 😂

[Laird Scooby]

Originally Posted by andy_d

either way you care to argue it
NO still remains the ONLY sensible choice,
the brain deads will NEVER change there minds , brainwashed they are

if you dont GET the point about YET ANOTHER BS restriction on what we can / cant do being a issue , then you ARE part of the problem
same as all the morons bleating about climate change being due to man
IDIOTS brainwashed by the gov / media to "justify" a whole raft of New taxations.

"give them an inch they take a mile" APPLIES to govment
take the Current BS whereby You HAVE no rights, are being controlled in what/where you can/cant do,, being made to wear Masks *that do NOT one single thing to stop corvid,, take the UTTER BS about the TRACK and trace crap being "to protect you" ,, it can NOT protect you ,, but you dumb brain deads swallow it,, THEN dare to have a go at people who THINK for themselves

the human race NEEDS a wake up call,, "google" doesnt have the answer you NEED to work things out for yourselves AND think for yourselves,, NOT have a go at those who DO and do NOT follow the brain dead heard,

Hell yeah!!!!!