Many people think that driving at 20mph lowers fuel economy. Yet research shows that for many cars a steady 20mph is the most efficient speed if you can keep it steady. Indeed at 20mph many cars will achieve fuel consumption better than 90mpg. But what changes things is in an urban environment where you are consistently slowing down for obstacles, lights, junction and congestion and then having to speed up again.
Here we explore the engineering principle behind the energy that gets used by a motor vehicle when moving.
The physics of automobile energy consumption are pretty clear. An engine has to use energy from whatever fuel it uses for the purposes of :-
Due to moving
a) Overcoming rolling resistance
b) Overcoming transmission friction and losses
c) Overcoming air resistance
Due to climbing hills
d) Increasing the Potential Energy in the vehicle due to altitude gained
Due to increasing speed
e) Increasing the Kinetic Energy in the vehicle due to the velocity gained
Rolling resistance and transmission friction are broadly linearly proportional to to speed.
Air resistance is proportional to the square of speed.
Potential energy is linearly proportional to the height gained.
Kinetic Energy is proportional to the square of the speed gained.
a) and b) are very similar for vehicles at 20mph or 30mph.
c) is proportional to speed squared but is relatively low at 30mph or below.
d) is not affected by the speed at which you climb a hill. That only affects the rate at which you use fuel per second, the total amount required to climb a given altitude will be similar regardless of speed.
e) is the one that determines urban fuel consumption the most. Getting a vehicle to 30mph takes 2.25 times that to get to 20mph (ie 30×30 compared to 20×20). Given that in most urban setting there is repeated slowing down and often stopping for hazards, obstacles, congestion, lights, it is the repeated depletion (braking) and then acceleration to peak speed that uses most fuel. That is why a 20mph limit cycle will require less than half the energy than a 30mph limit cycle for acceleration.
Whilst the sweet spot of the engine in terms of efficency may make a slight difference this is not significant.
So, the mechanics and physics are quite clear. Smoother driving to a lower limit will always require less energy, less fuel and produce fewer emissions than repeatedly accelerating to a higher limit.
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Facebook TwitterGetting to 30mph or more, is often just stupid but it’s what folk were taught to do in training… When experiencing difficulty and learning from it, wasn’t going to happen if all the traffic ahead disappeared into the distance.
If you’re qualified you should be doing it better. Maybe driving slower if going faster would be as wasteful or as pointless as tailgating in a three lane wide, traffic jam.
There’s an issue not mentioned… Braking. (To a lesser degree, simple deceleration in low gears from high speeds too), turns fuel already burned and used, back into heat. Which is wasted fuel. The less fuel burned to get to a common speed, without frequent or hard braking means greater distances travelled with a closed or almost closed throttle, with an air increase, metered by computer.
If average speeds in an area are low due to congestion and normal conflicts with other road users etc, then fuel used to get to, let’s say, 4 times possible average speed, is three lots of fuel wasted.
And arguing low gears wastes fuel, is pretty silly. Few if anyone has a fixed fuel jet carburettor with an enrichment jet anymore… Fuel injectors and computers are more than capable of metering low fuel at a steady throttle level at a low speed even if revs are 15% or so, higher.
My old semi auto, (it has a clutch-less robot manual type gearbox), knows if I’m going up or down a hill and if (by throttle pressure), I want more fuel or none at all… It usually gives me the right gear too and even seems to know it I have one or five people in the car as well…
It must do because on identical unhindered journeys within a few seconds timing of each other, the fuel consumption increases with extra folk on board but I’ve done nothing in particular, in automatic, to cause that increase.
I assure you that a car can do 20mph in 4th gear. I had experience of a lowly Honda Jazz doing just that. There is evidence that at 20mph 0r 30mph you will get about 90mpg. The difference between the two will be up to + or – 10% depending on exact gearing. See https://www.telegraph.co.uk/news/2109539/Fuel-prices-Slower-driving-can-save-motorists-500-a-year.html
“Mr De Nayer said: “There is a huge misconception that the most fuel-efficient speed is around 55mph. The study shows that the slower you go with the vehicle running smoothly, the less fuel you will use.”
A Citroen C4 1.6 diesel achieved 99.6 mpg at 20 mph, but just 29.3 mpg at 90 mph. A driver of an average car travelling 10,000 motorway miles per year would spend £518 more on fuel than if he had driven the same distance at 60 mph."
And, of course, CO2 emissions are measured in grms/km. So if the grms/km figure is the same for each then a 20mph car will be emitting only 2/3rd of the grms/second of a 30mph car.
I am afraid the “real world” is dominated by physics and engineering. To reach 30mph you need 2.25 times the energy to reach 20mph. And in a built-up environment that dominates fuel consumption.
1) TESLA has published a chart – I can send if you email me [email protected] – showing the range for their vehicles at different speeds. It peaks at about 20mph! This means that even for vehicles with no tailpipe emissions (tyre particulates are another matter, of course), slower speeds use less energy and bring less pressure on the grid.
2) While Gloucestershire CC is nominally in favour of 20mph (via its Vision Zero ambitions), it is not doing anything in particular to support its rollout across the county. We have an active countywide group campaigning for GCC to make it easier for communities that want 20mph to be able to get it. If you’d like to help in Stroud, I can put you in touch with the right people
but this isn’t about doing constant speeds, in an urban environment most driving includes a fair bit of stop and go – and traffic flows are generally more stabilised when drivers are forced to go slower – which is something that 20 urban limits very much help with.
so in real terms the fuel efficiency argument is a straw man when it comes to why 20 is plenty.
That fossil fuel efficiency argument put aside – within 7 years 80% of people will be in EVs – so not be driving ICE machines (that pollute more or less at any speed) at all… but with urban lower speed limits there will be fewer deaths and injuries from cars hitting people AND poisoning them with tail pipe gases.
EVs will be more efficient, the silly range argument will be destroyed by cars capable of doing 600+ miles on a single charge, and then we can concentrate much more pertinently on safer urban streets – where pedestrians have the rules on their side and cars cease to be tools for delusions of power to be acted out in the streets where people live.
I suspect that the answer to your question is that it all depends on your driving style and the frequency of stop starts. Modern ICE engines will do about 90mpg at a constant 20mph, but its the repeated acceleration that reduces this in urban conditions.
If your 37.3mpg is an average for an urban trip then that is pretty good. As it requires 2.25 times the energy to increase the Kinetic Energy of a car to 30mph compared to 20mph, then I suspect that your fuel mpg would be much less if you were in an environment with a 30mph max speed and subject to repeated stop/starts.
Electric vehicles will be subject to the same use of energy for transfering battery energy into Kinetic Energy. However this will be lessened by the usual regenerative braking. However this is only 60-70% efficient so there will still be some losses.
I trust that this helps.
Now the manufacturer advises it can achieve 54mpg combined urban/Highway, however in the numerous 20mph zones in Edinburgh it is achieving 37.3mpg, significantly less than the tests suggest. Do electric vehicles suffer the same real world dramatic drop that the Kinetic theory suggests does not exist. The car while in 3rd gear is at 1300rpm. So I would tend to disagree with this 19th century equation. I am looking at an electric vehicle, for urban use(utterly useless in real world motorway driving).
What car did 90mpg at 20 mph? Was it a Solar powered Hybrid with pedal assist?
1/ methodologically, it doesn’t control for the rate of acceleration, so the numbers would have very large CIs
2/ the paper includes the following description:
“4 Consumption Model Description
4.1 Average Speed Consumption Model Adjusted with Road Gradient
Average speed is a key factor in vehicles’ consumption and emissions output. Several studies have found that the relationship between the specific consumption and vehicle speed is U-shaped (Andre and Hammarstrom 2000; Smit et al. 2008). Therefore, it is possible to develop speed-based consumption factors (grams of fuel per kilometer) simply by using traffic flow records as input data for the emission model.”
I’d be interested in the research that you cite that finds 20 and 30 have similar economy levels for ICEs. I find that I’m usually in a different gear at the different speeds, so, assuming little difference in the engine efficiency, if the rpm is similar, then there ought to be a difference in gearing (m/revolution of the engine.)
Other reports show that when looking at steady speed then the fuel consumption at either a steady 20mph or 30mph is both similar and relatively low and is not the important factor in real world urban fuel consumption.
And as for speed bumps, we already campaign against them. Communicating with drivers via their buttocks and spines may be the only way to communicate successfully with some of them, but they revert to norm far too easily. That’s why we campaign for 20mph limits with signage, engagement and enforcement. Less stress for drivers, walkers and cyclists.
At low speeds, only a small fraction of the energy burnt is used to propel the vehicle. Most is turned into heat by the internal combustion engine. The shorter the time the car is producing that heat, the lower the energy used. So to improve efficiency you need to travel faster.
Going faster does increase rolling, mechanical and wind resistance but the balance point is around 55mph (vehicle specific).
I’m glad you realise that energy is wasted as cars have to avoid obstacles – can I suggest you campaign to remove the worst offenders, speed bumps?
You are right about motorways being fuel-efficient, but that is because they offer smooth and steady driving with no stopping/starting for hazards, crossings, lights, parked cars, bends, junctions, etc. If there were then your fuel consumption would be far greater if the peak speed between stops were higher.
To answer your questions :-
Where did you get your FACTS from ?
As I explained, they are engineering principles?
Who did your research?
Well the “research” comes from people like William Thompson, who later became Lord Kelvin. He is credited with introducing the concept of kinetic energy in 1849. We now associate the concept of an object’s kinetic energy with the quantity of one half of its mass multiplied by its velocity squared.
The research isn’t mine. Whilst I do have an Honours Degree in Automobile Engineering, I am just explaining what are well established principle.
What car did you use?
Clearly I what I explained applies to all cars.
Who did the measurements?
Re-read my article above and you will see why this is irrelevant.
And perhaps you tell us what car you have so that we can check the gear ratios in 2nd, 3rd and 4th.
Interestingly, the most fuel-efficient roads in the country are not quiet extra-urban dual carriageways or 20mph city streets, they are motorways. This is where you can leave the car in top gear and gently cruise along, using minimal fuel.
https://www.rac.co.uk/drive/advice/how-to/fuel-saving-tips/
1 At 30 mph the economy meter on my car is much higher than at 20mph. FACT
2 At 30 mph my car says I am doing 53mpg at 20mph its 44mph. FACT
3 At 20mph I have to go into high revs in 2nd. My car wont stay at 20mph in 3rd. FACT
Where did you get your FACTS from ?
Who did your research?
What car did you use?
Who did the measurements?
In the report you will find that : “The increase in fuel economy for speeds above 70 mph is likely due to the vehicle achieving high speeds while traveling down slope. Therefore, this increase in fuel economy is not expected to be characteristic of all travel at these higher speeds.”
This therefore demolishes the argument that the best fuel economy is over 85mph. Furthermore the report does not show whether these are instantaneous speeds or average speeds for a time or distance. Hence could be very much conditioned by lower speeds being due to start/stop conditions and hence not steady state.
See table 5.12 in https://tedb.ornl.gov/wp-content/uploads/2021/02/TEDB_Ed_39.pdf#page=161
In that report – the best fuel economy was found when over 85mph for trucks! It gets worse the slower you go.
For electric, as there is significantly less energy lost when not moving, slower is generally more efficient (down to very low speeds), but slowing / speeding for speedhumps does lose a lot more energy than constant speed (despite some energy recovery when gently braking).
Your analysis works for electric cars, but not combustion engine ones, where most efficient speed for a mid-sized car is 55 mph ( https://tedb.ornl.gov/wp-content/uploads/2021/02/TEDB_Ed_39.pdf#page=137 ).