National Speed Limit Petition, Sign in here, Closing date 9th September

[sludgeguts]

Nope. The average car is most economical (both on fuel and emissions) at 55-60 MPH, reducing the speed to 50 WILL increase emissions, as well as fuel use

Funny you should say that.
There's mutterings of reducing the speed limit in town & on residential streets in this area to 20mph and we had a leter in the paper...


"At a steady 20 mph most cars will do about 70mpg plus per gallon. A Citroen C4 will actually do 94 mpg.

Whilst the variation between steady 20 mph and steady 30 mph is about 10% either way dependent upon the car, what really consumes fuel is the constant accelerating to reach maximum speed after any obstruction or stop.

Hence a 20 mph limit will always produce less emissions than a 30 mph limit because it eliminates all the accelerating from 20 mph to 30 mph which otherwise would be consuming fuel.

Hence far from causing more pollution 20 mph speed limits substantially reduce it.

Best regards

Rod King
www.20splentyforus.org.uk "

 

[fenton_jd]

Funny you should say that.
There's mutterings of reducing the speed limit in town & on residential streets in this area to 20mph and we had a leter in the paper...


"At a steady 20 mph most cars will do about 70mpg plus per gallon. A Citroen C4 will actually do 94 mpg.

Whilst the variation between steady 20 mph and steady 30 mph is about 10% either way dependent upon the car, what really consumes fuel is the constant accelerating to reach maximum speed after any obstruction or stop.

Hence a 20 mph limit will always produce less emissions than a 30 mph limit because it eliminates all the accelerating from 20 mph to 30 mph which otherwise would be consuming fuel.

Hence far from causing more pollution 20 mph speed limits substantially reduce it.

Best regards

Rod King
www.20splentyforus.org.uk "

Which is what hacks me off about the council putting in 10 sets of traffic lights onto perfectly good roundabouts/interchanges.

 

[sludgeguts]

Which is what hacks me off about the council putting in 10 sets of traffic lights onto perfectly good roundabouts/interchanges.

Just the point of my reply to the above...
"So, from the roundabout at the bottom of the high street, we have to accellerate & stop 10 times within 1KM - so much for the Council's green policy"
I actually listed every stop/start & the distance between them, some were as little as 15m, 20m, 35m.

God knows, I've even tried driving the road frugally, gently pulling away, foot off the pedal to cruise to the next red light but it simply doesn't work. The lights stay red for far too long. Normally when I see a red light I decellerate & change down when required so I'm in the right gear to drive through as the lights hit green but it's just impossible to do with this stretch of road.

 

[Most Easterly Pandas]

Funny you should say that.
There's mutterings of reducing the speed limit in town & on residential streets in this area to 20mph and we had a leter in the paper...


"At a steady 20 mph most cars will do about 70mpg plus per gallon. A Citroen C4 will actually do 94 mpg.

Whilst the variation between steady 20 mph and steady 30 mph is about 10% either way dependent upon the car, what really consumes fuel is the constant accelerating to reach maximum speed after any obstruction or stop.

Hence a 20 mph limit will always produce less emissions than a 30 mph limit because it eliminates all the accelerating from 20 mph to 30 mph which otherwise would be consuming fuel.

Hence far from causing more pollution 20 mph speed limits substantially reduce it.

Best regards

Rod King
www.20splentyforus.org.uk "

What a load of bull crap!

A car will do a better MPG at 55/60 than at 20. Fact! It's all to do with dictance traveled at amount of fuel used to travel it when upto the speed.

 

[Eklipze3k]

What a load of bull crap!

A car will do a better MPG at 55/60 than at 20. Fact! It's all to do with dictance traveled at amount of fuel used to travel it when upto the speed.

They're comparing 20 to 30, not 20 to 55/60. A constant speed (any speed) will always be more economical than start-stop-start-stop though.

Also a car will have crap MPG at 55/60mph if it's designed to have it's best economy at an RPM better suited to doing 30-40mph, like a Seicento for example ...

 

[Most Easterly Pandas]

They're comparing 20 to 30, not 20 to 55/60. A constant speed (any speed) will always be more economical than start-stop-start-stop though.

Also a car will have crap MPG at 55/60mph if it's designed to have it's best economy at an RPM better suited to doing 30-40mph, like a Seicento for example ...

Yes, but no car will do that MPG at that speed!

Your sciento will be better at between 50/60, rather than 30-40, trust me

 

[sludgeguts]

What a load of bull crap!

A car will do a better MPG at 55/60 than at 20. Fact! It's all to do with dictance traveled at amount of fuel used to travel it when upto the speed.

Do you have any links to websites proving this theory?
And surely it's all to do with rpm at given speeds as well as rolling resistance and wind resistance?
I think you're confusing things. 55mph is probably the best top speed for fuel economy if you have to travel fast, above this speed, you're using far more fuel to keep up the speed than you have to.
Driving at 50 uses 15% less fuel than at 70.
Driving at 60 uses 9% less
Driving at 80 uses 25% more than at 70

 

[Most Easterly Pandas]

Driving at 50 uses 15% less fuel than at 70.
Driving at 60 uses 9% less

So lets assume 45 (50% of 70) uses 20% less fuel. 0.8l say compaired to 1.0l at 70.

So at 70 you do twice the distance as at 45, and use only 0.014L a mile, at 45 you use 0.017L

So ultimatly your using less fuel, but your getting alot less miles per unit of fuel

Simples.

It's that balancing out of how many miles per unit of fuel (normally done in Gallons) you can get, and what that maximum is. And this is normally done at 55MPH with the gearing in the average car.

 

[sludgeguts]

It really is that simpe is it?
You've completely disregarded the additional fuel consumption due to higher rpm and increased wind resistance.
And since when did you cover twice the distance at 70 than you would at 45?

 

[Most Easterly Pandas]

It really is that simpe is it?
You've completely disregarded the additional fuel consumption due to higher rpm and increased wind resistance.
And since when did you cover twice the distance at 70 than you would at 45?

Whoops, me tired, 35 MPH even.

Will redo calcs in morning, but yes it really is that simple. higher RPM will use more fuel, but if doubling RPM uses twice as much fuel but increases cars speed by 2.5 times then, less fuel over all is being used.

 

[The Beard]

No. People, and that includes most on here, only want to pass a driving test rather than learning to drive. If the Government wants to improve road safety then they should make Pass + compulsory and make it a minimum of 20 hours instead of 6.

and lessons are deer enouth already

OI! You nicked my idea. I've said the very thing on here several times.

Even the kid's parents think along the lines now of..."He'll really start to learn to drive when he passes his test. Just teach him the tricks of the trade and he'll pick up the rest as he goes."

The number of times over the last 22 years that I've turned up for a pupil's lesson after a snow fall only to be told the kid wouldn't be having a lesson as it was too dangerous. Mummy and Daddy didn't want the fruit of their loins out in the snow until he/she had more experience. When I'd ask how they were going to gain experience of driving on snow or ice, they couldn't really answer. The first time they'd experience a skid would be when they were on their own. The chances are the car would be well into the skid before the driver knew it.

A few weeks ago I went to Caernarfon for a couple of days. I took the M56 - A55 to Bangor then a short stretch of Dual Carriageway. On the way back I went via Porthmadog - Ffestiniog - Ruthin - Corwen and then Chester and the M56 home.

The average speeds were 65 to 75 on Motorways and Dual Carriageways and as fast as I could make it on National Speed Limit Dual Carriageway yet still stay legal. Overall average fuel consumption for the trip: 67.6mpg.

I think MEPs got a point in what he says. I've become a bit (lot?) anal about fuel consumption in the MJ. In a car like the Panda, wind resistance plays a large part, mainly because it's a boxy little thing. My average day to day fuel consumption is around 56mpg. That includes quite a lot of Motorway driving. Although 15 miles and 15 minutes seems a reasonable distance, the longer the
journey, the better the fuel consumption. I would think that the period of rich running at start up must have an effect.

If I get up to 70 and sit there, the petrol consumption improves very slowly. If I keep it at 65, it mproves at a greater rate. At 60 it improves at a similar rate. Down at 50 though, it appears to use more diesel. There must be some form of co-relation between speed and fuel used other than the simplistic answer that the faster you go, the more you use.

Experimenting one Sunday morning going to work and about 05:20, the fuel consumption improved by 0.3mpg over the 15 miles as I drove on at 50. The following morning, I drove at 60 and the improvement was about double.

 

[Most Easterly Pandas]

I think MEPs got a point in what he says. I've become a bit (lot?) anal about fuel consumption in the MJ. In a car like the Panda, wind resistance plays a large part, mainly because it's a boxy little thing. My average day to day fuel consumption is around 56mpg. That includes quite a lot of Motorway driving. Although 15 miles and 15 minutes seems a reasonable distance, the longer the
journey, the better the fuel consumption. I would think that the period of rich running at start up must have an effect.

If I get up to 70 and sit there, the petrol consumption improves very slowly. If I keep it at 65, it mproves at a greater rate. At 60 it improves at a similar rate. Down at 50 though, it appears to use more diesel. There must be some form of co-relation between speed and fuel used other than the simplistic answer that the faster you go, the more you use.

Experimenting one Sunday morning going to work and about 05:20, the fuel consumption improved by 0.3mpg over the 15 miles as I drove on at 50. The following morning, I drove at 60 and the improvement was about double.

:worship:

I've got it in my head, and know it all up there and it all is logical and makes sence, but can't put it into writing

Work than uni assignments

 

[Chas Uno]

So lets assume 45 (50% of 70) uses 20% less fuel. 0.8l say compaired to 1.0l at 70.

Normally figures are given as fuel used over distance covered i.e. mpg. Is that what you're trying to say? So at 45mph you are using 20% less fuel than at 70mph, over a given distance. That means that a car traveling at 45mph will take longer to cover the same distance compared with 70mph, but WILL ALWAYS USE 20% LESS FUEL OVER THE SAME DISTANCE!

So at 70 you do twice the distance as at 45, and use only 0.014L a mile, at 45 you use 0.017L

So ultimatly your using less fuel, but your getting alot less miles per unit of fuel

Simples.

Flawed logic Jon! If you use 20% less fuel at 45mph than at 70mph the lower speed will give the best mpg!

It's that balancing out of how many miles per unit of fuel (normally done in Gallons) you can get, and what that maximum is. And this is normally done at 55MPH with the gearing in the average car.

Jon, are you only going by the Government fuel consumption figures that quote averages for the urban, constant 56 and constant 75mph tests? You WILL get better mpgs at lower than 56mph by some simple logic, not least because the lower the speed the less the wind resistance you have to overcome. Higher speeds will increase wind resistance, which increases drag, which requires more power and thus more fuel to overcome. Wind resistance also increases exponentially with speed, which is why doubling the horsepower of a given vehicle won't double its top speed.

The lowest constant speed in the highest gear your car will pull, with the lowest engine workload and the least wind resistance, will give the best mpg. This is all covered by the laws of physics! You also need to bear in mind that the peak torque rpm is also often the most efficient rpm to drive a car for fuel economy though as above you have to factor in wind resistance along with overall gearing when determining the optimum speed for economy.

But put simply, a car that can travel at a constant 40mph in top gear (without labouring the engine) will always be more economical than the same car traveling at 70mph. And you don't even need to be Einstein to know that

 

[Most Easterly Pandas]

I'm just gonna give up now

 

[sludgeguts]

. There must be some form of co-relation between speed and fuel used other than the simplistic answer that the faster you go, the more you use.

Experimenting one Sunday morning going to work and about 05:20, the fuel consumption improved by 0.3mpg over the 15 miles as I drove on at 50. The following morning, I drove at 60 and the improvement was about double.

I used to drive a sharan & thought it was great on a run. Steady 70 in 6th gear, the trip reported high 60s mpg (the odd time I looked & average trip). When it came to filling, I discovered the figure to be arouind 45mpg.

I used to do a lot of motorway work in a 2.3 diesel pug pickup, paid by the hour, I quickly discovered my best economy came at a steady 56. Driving at 70 you could watch the fuel tank empty before your eyes.
That was without fancy trip computers etc but I did have a cruise control retro fitted (as I was doing thousands of miles on the motorway).
I used to own a honda civic 1.4 auto. Majority of driving was around town. If I drove it like it was something special, I'd see a return of 30mpg. This was often spoiled if I wanted a fast away from the lights where once or twice over a tankful would see a drop down to 25mpg.
Somewhere in my garage I have a vacuum gauge which links to the inlet manifold. It shows numbers for vacuum as well as coloured bands indicating best use of fuel. I used to have it on a petrol car & found it brilliant for getting good fuel consumption - especially on a run where you could keep to a constant speed by feathering the throttle - if you put your foot down, it really showed how much fuel you were consuming as the needle slammed over to the limit. I've noticed a few buses and lorries have a similar feature on their rev-counters, I have a green band between 2000 and 2500rpm indicating this is the best speed to keep the engine at for fuel economy. Sadly, as the engine is grossly underpowered for the weight of the bus, it is impossible to keep the needle in the green.

I expect there is a correlation somewhere out there but as every car has a different coefficient of friction as well as a different drag coefficient (etc etc etc) I suspect it would be impossible to provide a foolproof table.
Which does lead me to wonder why large vehicles are limited to 100kph (and that includes some transits!) which is about 61mph - and speed limits in America are mostly (if not all?) 55mph.

 

[sludgeguts]

I'm just gonna give up now

Before you go...
Have a read...

http://www.whatcar.com/car-news/fuel-economy-making-every-gallon-count/233124

"It's all down to wind resistance - the greater the wind drag, the more work the engine has to do, so barrel along at 100mph rather than 50mph and your engine will use roughly five times the fuel.

You'll cover twice the distance in the same time, of course, but that still works out to more than double the fuel for every mile travelled."


http://www.transportpolicy.org.uk/Future/20mph/20mph.htm


Interesting articles relating to speed/economy

 

[Most Easterly Pandas]

Before you go...
Have a read...

[URL]http://www.whatcar.com/car-news/fuel-economy-making-every-gallon-count/233124[/URL]

Right, I'm gonna admit it. Looks like I was wrong :worship:

Was an interesting read

 

[Most Easterly Pandas]

sludgeguts, can I invite you to here please

https://www.fiatforum.com/panda-new/201642-gear.html

 

[ChrisUK]

My coupes happiest at 80mph+ so I'll carry on doing it while safe to do so weither they lower the national speed limit or not. F***ing nanny state they'll tell me how and when to have a s***e next.

 

[Garree001]

F***ing nanny state they'll tell me how and when to have a s***e next.

They already do if you are in the south east during a water shortage! If it's brown and yellow and all that...