2011-10-01 13:18:44How to burn even more oil


The British government is considering raising the motorway speed limit to 80 mph and reducing the town limit to 20mph.

At speeds over 50mph cars become ever more fuel inefficient - said to be something to do with drag, but I don't see how clothing preferences come into it.

At 30mph most modern cars are close to tickover in top gear.

At 20mph they usually need to be in 3rd gear to keep the engine running smoothly.

Assume only 10% extra fuel in 3rd gear at 20mph.

Assume 5 liters per hour at 30mph, giving 5.5 @ 20mph.

The 20mph journey takes 50% longer so that's 8.25 liters as against 5 liters.

My estimates are very conservative.

Idiot drivers will be idiots at any speed: I would take away their driving privileges permanently.


Yeah, it's politics, but I'm only human - or so my doctor tells me.


Please discuss - the physics, not my alleged human body.  :-)

2011-10-01 18:23:48
Paul D


The problem is that if you make things easier, more people will use cars and it is that which will boost emissions.

Discussing only physics, ignores human (group/national) behaviour. The system isn't just how a car works, it is how people use the roads, cars, where they live, where employers base themselves etc.

2011-10-01 20:09:34



- "tickover in top gear" = ??

- not clear what you are trying to calculate

- simple model of air-friction force is proportional to (speed)^2


You might look at: http://en.wikipedia.org/wiki/Fuel_economy_in_automobiles#Fuel_economy_statistics

[edit] Physics

The power to overcome air resistance increases roughly with the cube of the speed, and thus the energy required per unit distance is roughly proportional to the square of speed. Because air resistance increases so rapidly with speed, above about 30 mph (48 km/h), it becomes a dominant limiting factor. Driving at 45 rather than 65 mph (72 rather than 105 km/h) requires about one-third the power to overcome wind resistance, or about one-half the energy per unit distance, and much greater fuel economy can be achieved. Increasing speed to 90 mph (145 km/h) from 65 mph (105 km/h) increases the power requirement by 2.6 times, the energy per unit distance by 1.9 times, and decreases fuel economy. In real world vehicles the change in fuel economy is less than the values quoted above due to complicating factors.

The power needed to overcome the rolling resistance is roughly proportional to the speed,[15] and thus the energy required per unit distance is roughly constant. At very low speeds the dominant losses are internal friction. A hybrid can achieve greater fuel economy in city driving than on the highway because the engine shuts off when it is not needed to charge the battery and has little to no consumption at stops. In addition, regenerative braking puts energy back into the battery.

2011-10-01 21:34:15


At higher constant speeds, resistance increases in a non linear fashion which is why CO2 emissions/km increases above 65kph (65 kph ~40mph  130kph ~80mph). However, at lower average speeds we perform a lot more stop-start driving which loses energy in the brakes, hence CO2 also increases at lower average speeds below 65kph.

(It is actually more complex than this due to varying IC engine specific fuel consumption at different loads and speeds, but that is the general rule)

Now the plan is to increase speeds at the top end, yet introduce more 20mph zones at the bottom end. So you can see it is going to increase CO2 in both cases!

The bizarre reason used to justify this is to "shorten journey times and help boost the economy"!

Read more: http://www.thisismoney.co.uk/money/cars/article-2041304/Motorway-speed-limit-rise-80mph-bid-boost-economy.html#ixzz1ZWsR5YEJ

2011-10-01 23:02:13
Alex C


Without knowing much about the average commute for a worker in the UK, a few questions come to mind:

- would increasing highway speed limits really significantly decrease the time it takes for one to travel to work?

- for those vehicles where travel time actually DOES impact their income (say, trucking), would they be able to drive at higher speeds while still maintaining a safety factor?

- since generated wealth is determined by hours worked (roughly), and not by the amount of time one actually puts into the working process (e.g. driving to/from), wouldn't we also need emplyers to extend their working hours for this to be more effective?  In other words, if the drive is shorter won't people just leave later and spend the same time at work, just more at home?

- wouldn't this decrease the amount of spending money that people have since they'll be paying for more fuel?  So, from a consumer standpoint this doesn't make much sense.

2011-10-01 23:03:49


Are you sure you're not misreading the article?

"Transport Secretary Philip Hammond has already made clear his concern that anti-car campaigners have for too long used road safety as a convenient excuse both to prevent the speed limit on motorways being raised and to push for more 20mph zones in urban areas, even where they are inappropriate."

- So they do want to push for higher speeds, to get faster trips. Yes, fuel economy will suffer; but that is a tradeoff.

- They are not proposing to lower speed limits in the city.

2011-10-02 09:52:44


perseus: spot on!  'twas ever thus, even in the days of steam: too fast or too slow burns extra fuel and wastes money.


neal: the revs of an average car in Britain at 30mph are slightly higher than tickover.  At 20mph they are at or below tickover.  I have most commonly found in testing that a car will only run smoothly in 4th gear at a minimum of about 31 or 32 mph.  Although a well set up car will just about run at 20 mph in 4th gear, it will not run smoothly and will lack power.

Some news reports are mangling what the government is trying to do. The Independent puts it correctly.

The speed limit on Britain’s motorways is set to rise to 80mph but with a big expansion in the number of 20mph zones in cities and towns, The Independent has learnt.

In the UK we have speed bumps and chicanes which are called 'traffic calming' measures.

Speed bumps cause damage to vehicles and driver frustration.

I have observed that drivers who want to complete a journey in a fixed time will, if forced to drive slowly in one place, drive much faster elsewhere.  All in all these proposed changes will increase our national fuel consumption by a very significant amount.  But hey! - CO2 is good for plants, right?

2011-10-02 18:49:11


Perhaps I should have said coalition, but this whole compromise was necessitated by the Tory proposal for a speed limit increase.  We know how coalitions and compromises make things even worse.  It is all only a proposal at this stage anyway.

Another part of the deal, to keep the Lib Dems happy, is understood to be the introduction of more 20mph zones in residential areas

Read more: http://www.thisismoney.co.uk/money/cars/article-2041304/Motorway-speed-limit-rise-80mph-bid-boost-economy.html#ixzz1ZbmAYkCx

Of course if there are more accidents, we could get a mixture of two separate modes, 80-90mph driving with intermittent stretches of 10-20mph at the accident black-spots!  So no-one wins.

2011-10-02 21:56:28
Paul D



- would increasing highway speed limits really significantly decrease the time it takes for one to travel to work?

In absolute terms no.

If I cycle into town, a car driver could do the same journey in slightly less time (5 minutes?), if you exclude finding a parking space.

You could say that is 100% improvement over cycling. I say it's a few minutes which increases your chances of ending up in hospital due to sedentary living.

Shaving off a few minutes off a motorway journey doesn't justify the change. In any case we all need to live and work locally, so motorways are by definition a problem.

2011-10-02 22:03:12
Paul D


The speed limit on Britain’s motorways is set to rise to 80mph but with a big expansion in the number of 20mph zones in cities and towns, The Independent has learnt.

Actually Tory MEPs voted against an EU policy to introduce urban low speed limits (30kph/20mph).
So it seems that the policy is OK as long as it doesn't come from Europe??

2011-10-02 22:04:44
Paul D


Speed bumps cause damage to vehicles and driver frustration.

Only if you drive a car. The simple solution is to not drive a car!
Then the speed bumps could be removed due to reduced traffic.

2011-10-02 22:15:06


I live in Munich, and I don't have a car. (Though many of my friends do.)

Don't know about folks living in London: I've been tourist there w/o. Outside of London?

In the US: Basically not practical to be an independent adult w/o a car. Even when visiting San Francisco for more than a week at a time, I rent a car; and if I'm staying outside SF, for sure.

2011-10-03 01:21:31
Paul D


I've had car drivers overtake me (cycling) a few yards from a road junction, only to stop at the junction, with me behind them. Admittedly many don't overtake. They know that the probability is that they won't get any where faster by overtaking me.

The junction that this happens also leads to another junction (with lights) a few yards away, so even if they quickly negotiate the first junction quickly, the probability is that they'll get caught by the second.