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Automobile fuel efficiency

Discussion in 'off topic' started by Wilson, May 15, 2019.

  1. SteveG

    SteveG pfm Member

    This is a description of what my car does: "Some vehicles, like the 2018 Mercedes Benz C-Class, take Eco Mode to another level entirely, by introducing a “Glide” capability within its Eco Mode. In “Glide” mode, the car decouples the engine from its transmission when you release the gas pedal, allowing the car to “glide” down the road. The moment you ease back on the accelerator, it re-engages – similar to pressing the clutch in a manual transmission vehicle."
  2. Stunsworth

    Stunsworth pfm Member

    So does mine. Unless it’s a steep hill the car will probably slow down. The question is will getting it up to speed use more fuel than idling in neutral would?

    I don’t idle in neutral for the safety reasons mentioned above.
  3. Wilson

    Wilson _

    Tony, our local government is planning to lower the limit on side streets from 30 to 20. I wonder what effect this will have on a driver's fuel economy.

  4. sean99

    sean99 pfm Member

    The problem with 30mph limits is that so many drivers do 35mph. 25mph is probably a good speed to trade pedestrian safety vs traffic flow, provided the drivers really drive at 25mph.


    "The average risk of death for a pedestrian reaches 10% at an impact speed of 23 mph, 25% at 32 mph, 50% at 42 mph, 75% at 50 mph, and 90% at 58 mph. Risks vary significantly by age. For example, the average risk of severe injury or death for a 70‐year old pedestrian struck by a car traveling at 25 mph is similar to the risk for a 30‐year‐old pedestrian struck at 35 mph."

    Most cars reach peak efficiency at around 45-55mph, so in terms of gas consumption / greenhouse gas production there's not a great deal of difference between 30mph and 25mph - if you're concerned about urban pollution then regenerative braking is the answer, either as hybrid or pure electric vehicle.
  5. TheDecameron

    TheDecameron Unicorns fart glitter.

    Edinburgh city centre is all 20mph now. The main effect seems to have been to deter people driving at 40mph. Most are keeping to 30 now.
    Tony Lockhart, sean99 and Wilson like this.
  6. Derek Wright

    Derek Wright pfm Member

    I had a Saab 96, it had a free wheel option, once you released the accelerator the engine would go to tickover and the car would go on and on. A friend had one as well and during the great petrol shortage when the national speed limit was 50mph (to conserve fuel) he was coastng down a hill at significanty more than 50 mph and he was stopped by the police who gave him a lecture about speeding and conserving fuel. He tried to explain that the faster he went down the hill the less time the engine would be using fuel.

    Driving with a freewheel gave you great practice at safer driving as you tended to leave a greater gap in front of the car to allow for the greater braking time, one was always braking from the actually speed with no engine braking being applied during time to move the foot from the accelerator to the footbrake.

    Happy days.
  7. stevec67

    stevec67 pfm Member

    Probably none. It's not about fuel economy, it's about killing fewer pedestrians in resi areas.
    Tony Lockhart likes this.
  8. -alan-

    -alan- pfm Member

    Yeah - if you're talking about side streets, the amount of time the cars will spend at a steady state 30mph or indeed 20mph must be minimal. Given that the driving will be mostly stop/start with lots of acceleration and braking (albeit gentle), hard to imagine there'd be any measurable change in average mpg.
  9. Wilson

    Wilson _

    Thanks, guys.
  10. Tony Lockhart

    Tony Lockhart pfm Member

    I think we should all forget advances in the finer details of internal combustion engines. EV and hybrid cars will make a mockery of everything.
  11. George J

    George J Herefordshire member

    I am really relishing the thought of having a real Electric Vehicle.

    It would only have to have real life range of sixty miles. Bigger trips could easily rely on trains ...

  12. Tony Lockhart

    Tony Lockhart pfm Member

    “Rely on trains”

    Well, not me.
    cutting42 likes this.
  13. cutting42

    cutting42 Heading to Fish Hacker Erg \o/

    I love trains and ride whenever possible. I live on the Tube and the WCML but still find it hard to get some places by train for sensible times and reasonable cost.

    That said, I am looking forward to getting an electric car in the next couple of years but will hire a regular car when I need longer journeys holidays etc.
    Tony Lockhart likes this.
  14. Minio

    Minio pfm Member

    I know that Wilson has mentioned petrol cars in the original post but as an aside I’m pretty sure my diesel will give very high fuel efficiency at 20mph.

    I base that on 1) the pesky computer always shows higher instantaneous mpg at slower speeds.
    2) At 20 mph the car is at idle speed in third gear with virtually no foot on the right pedal!
  15. Yank

    Yank Bulbous Also Tapered

    Indeed. Below 45 or so, frictional losses dominate. Above that speed, aerodynamic losses come into effect.
  16. Tony Lockhart

    Tony Lockhart pfm Member

    20mph zones are only small when compared to the journeys that cars should be used for most often. I really wouldn’t worry about the mpg in them unless you’ve a Chrysler hemi throbbing away up front.
  17. sean99

    sean99 pfm Member

    Unless you mostly drive for pleasure during off hours or live in the back of beyond (I think perhaps you do ;)) I would guess that more than half of the distance covered by many peoples' cars would be stop and go with an average speed of 25mph.
  18. Mike Reed

    Mike Reed pfm Member

    Had a teaching colleague in the early seventies with an old Saab on car share. He used this a lot and it put the fear of God up me as it wasn't a secure feeling at all as a passenger.
  19. stevec67

    stevec67 pfm Member

    I should be so lucky. If I can average 20 mph in town it's probably the middle of the night. Average for my 100 mile weekly commute is probably 40 mph, of which 60 miles is on motorway or dual so occasionally getting to 70.
  20. Tony Lockhart

    Tony Lockhart pfm Member

    In which case they’re firstly not going very far (‘miles’ per gallon) and secondly even if the limit was 50 it’d make no difference to the actual speeds achieved.

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