Do emissions and fuel used increase with 20mph limits?

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.

Showing 21 reactions

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  • Rod King
    commented 2023-05-14 08:31:17 +0100
    Hi Jeff

    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.
  • Jeff Angus
    commented 2023-05-13 17:06:02 +0100
    I have recently bought a new Vauxhall Astra 1.5 turbo Diesel Automatic. The car has the facility to change speed according to road signs. and limits the speed to 20mph regardless of how far you press the accelerator.
    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).
  • chris p
    commented 2023-05-05 14:30:56 +0100
    @Rod King its a Volvo v40 diesel. Will not do 20 in 3rd gear.
    What car did 90mpg at 20 mph? Was it a Solar powered Hybrid with pedal assist?
  • Tim Coote
    commented 2023-05-05 10:27:37 +0100
    I doubt that the measures represent instantaneous values during acceleration – although of course, they could – for two reasons:
    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.)
  • Rod King
    commented 2023-04-19 19:00:37 +0100
    Thanks Tim. I cannot be sure, but I believe that the graph you pointed to in the report was showing the average instantaneous fuel consumption at various speeds in drive cycles rather than the average fuel consumption at a steady speed. In the former case then the lower speed points would be more likely to be when the car was accelerating and hence using more fuel than at the same speed in a steady speed. This would persist until the aerodynamic drag become far more influential.

    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.
  • Tim Coote
    commented 2023-04-17 17:59:00 +0100
    @Rod King feel free to comment on the ResearchGate paper that I linked to. For ICE, the market has incentives to optimise fuel consumption at around 56mph. As an auto engineer, I’d have thought that this was well known, and how the trade-offs between engine performance and wind resistance play into this. Feel free not to worry about overcomplicating the message. I don’t have a degree in automative engineering, but I do have an MA in Natural Sciences and Computer Science.
  • Rod King
    commented 2023-04-17 08:04:41 +0100
    That 55mph may be a “balance point” for interstate highways, but it balances journey time v fuel used. In urban situations it is irrelevant.

    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.
  • River Rock
    commented 2023-04-15 22:58:56 +0100
    @Rod kinetic energy?
    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?
  • Rod King
    commented 2023-04-15 08:09:25 +0100
    Its mechanics and physics, Chris. Just like 1+1=2, Kinetic Energy= 1/2 mass times velocity squared.

    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.
  • chris p
    commented 2023-04-13 12:54:39 +0100
    The RAC says
    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.
  • chris p
    commented 2023-04-13 12:44:20 +0100

    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?
  • Bernard Sutton
    commented 2023-03-29 17:33:52 +0100
    i use 50% more fule at 20 mph than 30mph ie 2nd gear then 4th gear [ why lye bad }
  • Alex Hosking
    commented 2022-07-30 01:45:38 +0100
    This is based on people driving in a way that almost no one does in reality.
  • Rod King
    commented 2022-07-24 21:54:12 +0100
    Thank you River Rock

    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.
  • River Rock
    commented 2022-07-18 12:08:37 +0100
    For large trucks, the faster they go, the better the fuel economy due to energy lost by their engines (mainly by heat).
    See table 5.12 in
    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).
  • David Preedy
    commented 2022-07-18 11:32:46 +0100
    To what extent is the kinetic energy saving reduced by electric/hybrid vehicles which capture (some) kinetic energy to re-charge whilst braking?
  • Tim Coote
    commented 2022-05-22 20:30:25 +0100
    The research on fuel consumption vs speed seems to show that the engineering loss differential is larger than you’re claiming. ( .e) is only relevant if a vehicle is oscillating between <20 and 30mph, which may be the case in heavy traffic with some over-competitive drivers, but that’s hardly a reason to constrain use at other times.
  • Carol H.
    commented 2022-02-24 21:27:59 +0000
    I’ve been curious about the latest research on fuel emissions since the boom in electric cars. I rarely drive above 20mph but I live in a dense urban part of Hamilton, Ontario. I recently got my electric car tuned up at and I feel really good about sending less emissions into the atmosphere. Thanks for sharing what you’ve found.
  • Rod King
    commented 2021-08-09 16:15:02 +0100
    I think that River Rock forgot to say that the graph quoted only starts at 45mph and therefore says nothing about fuel economy at steady state speeds below 45mph. It it had done then it would have shown results in the region of 80-90mpg and hence far more efficient than at 55mph.
  • River Rock
    commented 2021-08-09 14:52:53 +0100
    Um – did you forget to include the 70% wasted energy that a car engine loses, due to various losses (mainly heat )?
    Your analysis works for electric cars, but not combustion engine ones, where most efficient speed for a mid-sized car is 55 mph ( ).
  • Will Watson
    commented 2020-02-12 11:38:42 +0000
    When I cycle I find there is a massive difference between wind resistance at 20 or 30 mph…