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Automobile fuel efficiency

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

  1. Wilson

    Wilson _

    Is the average petrol powered road car more fuel efficient at 30 mph or 20 mph.

  2. stevec67

    stevec67 pfm Member

    Usually 30. The lower energy requirement at 20 is outweighed by the fact that it won't hold top gear at 20.
  3. Rana

    Rana pfm Member

    Rule of thumb is a naturally aspirated or mechanically supercharged gasoline engine is most efficient at between 1500-2200 rpm and around 60-80% of full load at those speed. Turbos are can alter this depending on chosen compression ratio and turbocharger speed matching. So basically choose the highest appropriate gear safe for the situation, and keep engine speed at within 500rpm of 2000rpm for best economy. A modern multi-speed auto will do this for you. All of this goes out the window for older cars if you vary the throttle input too much, as this will add transient fuel enrichment, although modern calibration techniques and proper hybridisation can help minimise this.
    Rockmeister and Wick like this.
  4. davidsrsb

    davidsrsb pfm Member

    Modern cars usually have far to powerful engines for optimum economy at most legal speed limits
  5. sean99

    sean99 pfm Member

  6. Tony Lockhart

    Tony Lockhart pfm Member

    You are, in my opinion, wrong.

    Engines have extremely complex management of fuelling (timing and quantity), ignition and boost. Many very powerful cars are very economical when driven carefully. Their maximum power just doesn’t come into the equation.

    “Most” cars sold in Europe, at a guess, have engines in the 90-150bhp range. Considering the variety of uses they’ll be put to, none of them can be considered too powerful.

    @Wilson why are you asking? Concerned about fuel economy in 20mph zones? Don’t waste your breath! The subject has been done to death.
  7. zippy

    zippy pfm Member

    ... and to those of you who depress the clutch and coast up to junctions etc, that's actually using more fuel than if you leave the clutch engaged (due I think to clever engine management systems)
  8. TheDecameron

    TheDecameron Unicorns fart glitter.

    Up to about 15 years ago when I still drove manuals, I’d flip it into neutral and coast to the lights/roundabout at the end of a fast road.
  9. Tony Lockhart

    Tony Lockhart pfm Member


    I’d ask to get out of the car and bloody walk.
  10. TheDecameron

    TheDecameron Unicorns fart glitter.

    At that speed, you’d get a proper deglazing.
  11. davidsrsb

    davidsrsb pfm Member

    In that case why do hybrids give so much better economy from a relatively small petrol engine with the electric assist?
  12. Mike Reed

    Mike Reed pfm Member

    Why would anybody do that? Very strange driving behaviour, that.

    For 14 years this was also my practice when I went all over the country annually. What I didn't know then was that my first diesel and fuel injected wagon saved fuel on the overrun. Swings and roundabouts, I s'pose, as I wouldn't have coated as far on overrun. I remember a stretch on the A465 on the edge of the Brecon Beacons where the distance thus travelled was over two miles.
  13. Stunsworth

    Stunsworth pfm Member

    It depends. If you leave the car in gear and take your foot off the accelerator most modern cars will use no fuel. However the engine will act as a brake slowing the car down. If you de-clutch and coast, the engine will consume fuel at tick over revs, and the car will coast a lot further.

    I don't coast in neutral, but my guess is that would use less fuel overall than coasting in gear.
  14. cutting42

    cutting42 Heading to Fish Hacker Erg \o/

    Depends what you do with it. Top Gear compared a Prius with an M3 round a race track and the Prius gave worse fuel economy for the same laptime.
  15. deebster

    deebster pfm Member

    Hypermiling is my new thing, being old and tight, and when I fill the car up and coast downhill on the way home my just-reset MPG readout is 99.9, meaning no or very little fuel is being used.

    My dad, who as you'd expect is older but not so tight (and stubborn) always slips into neutral and coasts ('what I've always done kid'), so to take my mind off telling him for the 387th time it costs fuel I watch his MPG readout and it goes down.

    So, in my experience staying in gear is better. And in addition, in neutral you have no engine breaking going on, so if you do need to slow down abruptly you are relying purely on the brakes so stress them more than is needed.
  16. sean99

    sean99 pfm Member

    Plus, if for some reason the engine did stall while coasting you'll lose power steering and brakes. I once had a power steering pump fail while negotiating a roundabout. It was an "interesting" experience that I hope not to repeat.
  17. cutting42

    cutting42 Heading to Fish Hacker Erg \o/

    Coasting is a very bad idea from a safe driving perspective as well and would earn you a fail in any driving test.
  18. bor

    bor queue jumper

    I agree with the mighty Rana, 2000RPM part-throttle is the sweet spot.

    Re. Coasting - Most modern cars with an auto gearbox will coast in neutral with the engine idling if the car has an ECO mode option.

    The next development of this is to switch the engine off completely during the coast-phase and then use the stop/start system to re-start when required.

    I don't know how much fuel this saves, but tried to mentally count how many seconds per minute the car goes into coast mode on a busy motorway, and it was surprisingly high.

    I could believe a saving of 10% under those conditions.
  19. SteveG

    SteveG pfm Member

    In our A180 petrol Merc if you've got economy mode selected it drops out of gear and coasts when it thinks it's appropriate to save fuel. Not sure how it decided whether to coast or engine brake but it seems to work fine in practice.
  20. Minio

    Minio pfm Member

    My car quite frequently does 999 mpg when going down hill. In gear, right foot off.

    That’s what the computer says.

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