Sixty mph?

Question

My diesel car has a trip computer. When driving at 80 mph (not in the UK, obviously) the fuel consumption is ~40 mpg.
When driving at 60 mph, ~70 mpg.
Surley, to cut carbon dioxide emmissions, the national speed limit should be reduced to 60 mph on motorways, and enforced?

I think there is confusion here, congestion causes lower speeds, not the other way round.

Ken E - yes of course, but do you really think 70 mph has been set with science in mind? It's just a number.

Answer 1

I would quess that you have a small engined diesel car that doesn't have the power to maintain 80 mph efficiently.
You wont get many votes for a reduced and enforced slower speed limit on motorways.
We have too many vehicles on the roads these days and with everyone wanting to drive in their own way.
I like to cruise along at 60 which means I don't go in the outside lane much but if someone wants to go fast it doesn't matter what the speed limit is they are going to speed anyway.
Cars are much better at going fast/braking today than they were 30/40 years ago,so the 70 mph could be increased.However the cars are better but are the drivers?
So just relax don't hog the middle lane and carry on at whatever speed you're happy with and let everyone else speed past.

Answer 2

Interesting. In principle yes. In fact 0 mph would do the trick nicely...

However:

(1) at lower speeds, congestion tends to increase, which is not good for petrol consumption - in fact congestion is not good for anything except maybe reducing accidents?

(2) even without congestion, the lower the speed limit, the fewer people use motorways - you may find this beneficial for the environment, however they simply take other roads where petrol consumption rises because of the stop and go / accelerate and slow down pattern of driving, and incidentally the risk of accidents is higher

I think that road charging and more efficient cars are the best direct ways of reducing carbon emissions from motorway traffic. Indirect measures include improving public transport, but that is a longer story.

Cheers

Answer 3

It has been established that most cars get their best gas mileage between 55-65 MPH depending on factors such as drive axle and gear ratio, wind resistance, driving style and engine torque curve characteristics.

Here in the United States, the national speed limit used to be 60MPH. Also, in the eighties, a lot of car manufacturers put 55MPH right at the top of the speedometer since that was the national speed limit at that time. There was a study done that suggested drivers tend to keep the needle in the middle.

Yes, the speed limit should be reduced for environmental reasons. However, people tend to drive with the flow of traffic, and during rush hour people just want to get home. Also, I couldn't imagine during trips driving at what seems like a snails pace (60MPH) when I'm used to driving at least 70. If the speed limit is reduced after all this time, drivers will get angry. There are a lot more drivers than police officers.

Here in America, we love our cars and we love to drive fast. Our oil consumption shows, we have little regard for the environment when it cramps our lifestyle. This is sad for mother Earth, but it's our way of life. You can't convince the gearhead to trade his Camaro in for a Prius.

Answer 4

It is true the a car uses less petrol per gallon at around 55 mph (depending on the car) than at 70 or 80 mph. It is not true that the slower you go the better the fuel economy though. For instance, if you did a whole trip at 20 mph you would find that fuel consumption actually was higher than at 80 mph. There are lots of reasons for this.

Firstly, engines have an optimum efficiency. Have you every ridden a bycicle? Do you notice that if you pedal very fast in a low gear you seem to run out of steam very quickly without getting very far - and if you pedal very slowly each push seems to drain your legs - but if you pedal at just the right speed it seems to take very little effort. The same is true of engines and the revs they use. Engines like to rev at about 2500 rpm. Rev too high and you spend a lot of energy for nothing - too low and all the fuel in each revolution does not burn properly (so the engine "pinks").

Also - moving very fast through air means you have to push the air out of the way faster and with more energy - move slower and you don'e need as much energy to push the air out of the way. This effect increases in a curve with the biggest difference happening the faster you go.

Finally - moving faster often means that you have to break more often, and each time you slow down you loose all the energy you used to accelerate to that speed (by the way, accellerating more slowly relaly helps fuel economy). Driving a little more slowly tends to mean that your stay closer to your average speed and reduce the need for more fuel-thirsty acceleration.

S0 . . . congestion really does use up the most fuel, stopping and starting, accelerating and slowing, running the engine at a low efficiency. So do high speeds through wind resistance, pushing the engine at high revs and more variations in speed. You are right to travel slower and save your self money and the environment as well. However I don't think a lower speed limit would be observed by the BMW/LEXUS/SAAB/MERC crowd - do you?

Answer 5

Wow thats one strange car you got there. What does it do at 50 mph? 90 mpg? If what your saying works on all cars we could reduce the national fuel intake dramtically by cutting limits. But I run a fleet of 30 diesel cars and none of them have such dramatic fluctuations. I would get yours checked at a main dealer with diagnostic equipment. It must be a small car with a 6 speed box I guess. We have a lot of 1.3 cdti Corsa's which can't touch your mpg.

Answer 6

Don't be ridiculous, it should be raised to at least 80mph.The ridiculous 70 limit was started in the 60s when cars took a long time to stop and were not so safe.Cars nowadays can stop much quicker and safer so 70mph is stupidly slow.
As for cutting CO2 emissions (one m)..this country is so small in comparison to say China ,India and USA that if we stopped ALL the cars completely it would be like a gnat's piss in the ocean.It would not make any discernable difference.
Having said that if you've got a diesel car you can't enjoy the driving experience much.

Answer 7

Are you trying to tell me that at a constant 60mph versus a constant 80mph you save 30mpg. That's some car kid...
You use far more fuel around town than you do on the motorway. Your argument doesn't stand up. A car is at its most fuel efficient when its running at a constant speed/RPM.

Answer 8

There is much more to national speed limit than just emissions.

Different cars have different most economical speeds. BTW, yours is probably even lower than 60, did you try 50, too?

And overall, I believe speed limits need to be eliminated, not adjusted in any way :)

Answer 9

You presumably own the car and the fuel. What you choose to do with them, within reasonable limits, are no business of government even though they probably own the road. Engineers have for one reason or another decided that the safe speed of the motorway is X and within that limit you are entitled to do what speed you like.

Answer 10

As daft it may seem, if everybody drove at 60mph on the motorways, during peak times. Say 0700-1000hrs and 1530-1800hrs, the traffic would flow more smoothly and FASTER than it is at the moment. With speeding and sharp breaking, which causes jams.
I am a courier, self employed. And to conserve fuel, money to me(!), I always drive at 60mph on motorways. You just would not believe how less stressful it is too! I feel safer, much safer aswell.

Jojotraveller, I'm sorry, you are wrong. The complete opposite is true. If you've ever driven on motorways, where they have Overhead instructions, M42, M25..., you would have seen this proved correct.