Speed limits and petrol consumption...

Okell
Okell Posts: 568
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I recently attended a Speed Awareness Course.  (35 in a 30!  :/  )

Something we were told by the presenter that really surprised me was that with modern* cars you should not drive in "too high" a gear in 20 and 30 mph limits otherwise your fuel consumption will go up as the ECU will think that you are going too slow to be using that gear, and will try to increase revs.  (Or at least that's a paraphrase - possibly a poor one - of what I understood we were being told.)

Is that true or have I completely misunderstood? 

I seem to recall that when I first learned to drive in the late 1970s there was an emphasis on making sure you "matched" engine speed and gears to road speed, but I didn't think that was a thing any more.  For example, I was originally taught to sequentially change down through the gears when reducing speed to come to a halt, but I didn't think that that technique was taught these days.

I tend to drive in as high a gear as I can in 20 and 30 limits, and the instantaneous fuel consumption figures seem to show I'm using less fuel.  Not that I necessarily think those figures are particularly accurate or reliable.

I suppose what he might have been trying to suggest - without explicitly saying so - is that the higher the gear you are in, the more likely it is that your speed will inadvertantly creep up and you will be more likely to break 20 or 30 mph urban speed limits?


*Not sure what the definition of "modern" would be.  Would it include a 12 plate Mondeo 1,6L Ecoboost?
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  • Car_54
    Car_54 Posts: 8,103
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    Okell said:
    I recently attended a Speed Awareness Course.  (35 in a 30!  :/  )

    Something we were told by the presenter that really surprised me was that with modern* cars you should not drive in "too high" a gear in 20 and 30 mph limits otherwise your fuel consumption will go up as the ECU will think that you are going too slow to be using that gear, and will try to increase revs.  (Or at least that's a paraphrase - possibly a poor one - of what I understood we were being told.)

    Is that true or have I completely misunderstood? 

    Sounds dubious. Unless all those minicab drivers have got it wrong.

    I seem to recall that when I first learned to drive in the late 1970s there was an emphasis on making sure you "matched" engine speed and gears to road speed, but I didn't think that was a thing any more.  For example, I was originally taught to sequentially change down through the gears when reducing speed to come to a halt, but I didn't think that that technique was taught these days.

    You're right. That hasn't been taught for about 40+ years.


    See above.
  • Krakkkers
    Krakkkers Posts: 1,082
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    I have a V twin motorbike with a 100 mph first gear and around town 20 to 30 mph means revs so low even in first gear that the engine is either too lumpy and mechanical or you have to slip the clutch.
  • If you are in a high enough gear that you are lugging the engine then fuel efficiency plummets, but it gets worse the closer you get to lugging as well. The most efficient band for a petrol is between 1,500 and 2,500 depending on engine specifics, load etc. and for diesel 1,200-2,000. 

    Less important in the 20-30mph range, but as a general rule drag in air increases with a square of speed, the difference between 60mph and 80mph can easily be a 50% increase in fuel consumption. 

    Most modern (last 15+ years) cars will generally tell you if you are in a too low, or too high a gear.
  • daveyjp
    daveyjp Posts: 12,370
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    edited 12 October 2023 at 8:41PM
    1970s was 4 speed gearboxes, no ECUs and 8v engines.   4th would be good from 15-20mph to whatever top speed was (it certainly was in the Fiesta 1.1 I was driving back then).

    Modern gearboxes have 5-6 gears so you need to know what gear is best for the limits.  For 30 it will generally be 3rd or 4th, what you don't want is a labouring engine.

    Our Yaris sweet spot is about 1700 rpm which just happens to be 20mph in 3rd, 30mph in 4th, 40mph in 5th and 50mph in 6th.

    Its no different to pedalling a bike, cadence is key, keeping the pedals turning without using too much energy and you do this be selecting the right gear.p, too high or too low and your legs will let you know!
  • Johnmcl7
    Johnmcl7 Posts: 2,816
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    I've noticed with newish cars that when they have a gearshift indicator they often want to keep you in higher gears than I would normally.  I was struggling up a steep hill in a rental Kia Picanto 1.0 in second or third gear and the car was telling me to shift into 5th gear...the only way that car would get up the hill in 5th if it was being towed.
  • 400ixl
    400ixl Posts: 2,587
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    As someone has put, there is an optimal rev range. You don't want to be driving in a gear where the revs are below 1,200 really as it will put extra load on the engine and if you needed to accelerate quickly to avoid something the engine would not be as responsive. Equally you do not want to be in the 2,000+ range as that is just not efficient.
  • badmemory
    badmemory Posts: 7,485
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    Okell said:

      For example, I was originally taught to sequentially change down through the gears when reducing speed to come to a halt, but I didn't think that that technique was taught these days.

    When they taught that brakes had a nasty habit of failing.

  • tacpot12
    tacpot12 Posts: 7,829
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    The ECU will only try to increase the revs in a manual car if you are in such a high gear that the engine is turning at less than it's programmed idle speed. Other than that, the ECU won't try to increase the revs, but as others have pointed out, if you push the accelerator pedal with the car in too high a gear, you will get rubish fuel economy because you are in the wrong gear. This is basically what the instructor was saying. 

    You should be in a gear where the car can accelerate cleanly IF you want it to accelerate. If you are just pootling along in traffic or descending down a hill, you are fine in a high gear. The ECU will not try to increase the revs. (The revs may rise anyway as the ECU tries to keep the engine above idling speed. Engine braking is no longer recognised as a good technique because gearboxes cost a lot more than brake pads. Goind down hill, you shoudl be in a high gear and use your brakes to avoid building up speed. Only on very long descents where the brakes might overheat should engine braking be used. 

    Going down through the gears is no longer taught, for the above reason - if you are not accelerating you want to be in a high gear, when you are approaching the point where you need to accelerate or control your speed precisely, you are now taught to change to the correct gear for your road speed at that point and to go directly to that gear, not down all the intermediate gears because this takes too long and wastes fuel. 
    The comments I post are my personal opinion. While I try to check everything is correct before posting, I can and do make mistakes, so always try to check official information sources before relying on my posts.
  • Goudy
    Goudy Posts: 1,408
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    edited 13 October 2023 at 6:41AM
    If you put a modern car in gear and start lifting the clutch slowly without touching the accelerator, they will usually start increasing the rpm itself to try and prevent a stall.

    The same thing can happen if you lug the engine at slow road speed in too high a gear.
    The rpms drop below it's desired target when it knows the car is in gear so it tries to prevent a stall and you get that lurchy judder and a load of racket from the driveline as it's slops about as it tries to speed itself up a little.

    When this happens you might now try to accelerate out of it without changing down a gear or two.
    You are now asking a lot of the engine, moving a large mass like this in too high a gear from a slow speed is very thirsty, so your mpg gets hammered. 

    As already mentioned, there's a sweet spot in the engines power where it produces peak torque and the idea is to use the gears to keep it in that sweet spot as much as possible as it's most efficient.

    Over the years we've seen cars with 3 and 4 gears, then cars with overdrive, then as time went on cars 5 and 6 gears.
    More modern cars, particularly some autos can have 7, 8 or 9 gears.
    They are obviously not all spaced as the old 3 and 4 speed gears but spaced closer together to help keep the engine in it's sweet spot more often at different speeds, which helps with fuel economy.

    Again it been mentioned that some modern manual cars have gear shift indicators that help the driver select the correct gear at the correct time.
    Quite often drivers stare at it and usually disagree with it but it is set to try and keep you in that sweet spot as much as possible.

    If you haven't a shift indicator, all isn't lost as you can look up the rpm that peak torque is delivered from you engine easy enough.
    Mine produces peak torque between 1600rpm and 3200 rpm, so if I tried to drive it between those figures on the rev counter it should be at peak efficiency, but I don't really need to worry too much as it's a 7 speed dual clutch and tends keep itself in the sweet spot unless I really clog the accelerator and hit the kickdown.
  • born_again
    born_again Posts: 13,493
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    Goudy said:
    If you put a modern car in gear and start lifting the clutch slowly without touching the accelerator, they will usually start increasing the rpm itself to try and prevent a stall.

    Once car is moving keep changing gear (don't touch throttle) & you will find the lowest speed your car will do in each gear at tickover. Works great with modern FI.

    Very handy in slippery conditions. Just slowly lift clutch & car will move itself.
    Life in the slow lane
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