cruise control kills MPG

Discussion in 'Gen 4 Prius Main Forum' started by Kindke, May 29, 2021.

  1. Kindke

    Kindke Member

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    Since upgrading to the gen 4 ive been thoroughly enjoying the cruise control however after some experimenting Ive been horrified to discover it totally kills the mpg.

    on 50mph speed limit roads the cruise control will usually get around 65-68mpg but when controlled by myself I can quite comfortably get over 75mpg

    This is disappointing and totally unacceptable as im a mpg maximalist and want to squeeze my petrol efficiency at all times.
     
  2. Mendel Leisk

    Mendel Leisk Senior Member

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    The above mpg values are per imperial gallons? For mpg (US) you need to multiply by 0.832674.

    The 65 mpg (IMP) for example, becomes 54 mpg (US). Not bad for 50 mph?

    I think you’d need at least a tank’s mpg, say one where you consistently use cruise (whenever practical), and another tank where you don’t.
     
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  3. meeder

    meeder Active Member

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    Cruise control is more for comfort than economy.
     
  4. sam spade 2

    sam spade 2 Senior Member

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    I see no problem here.
    Except that you are unwilling to pay a small penalty (15%) for a little convenience.
    The solution is simple: If fuel efficiency is an overriding requirement for you, just don't USE the cruise control.
    End of story.
    Note: That situation is NOT unique to your model of car. Most ALL of them will show similar numbers for those with an obsession.
     
  5. robsnyder20

    robsnyder20 Active Member

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    I am an avid user of cruise control. There are a few tips I can share. DON'T let the car decellerate at a light, ect. It will force the car to rev up which I can tell because of my scangauge. Also you are loosing regenerative braking when allowing the car to brake. While it does give you some back the car by default revs the engine to decelerate instead of using all regen braking. Also, try tapping the brake when coming up to a light to slow down before needing to. This will give you more regen braking and also possibly the light might turn green before coming to a full stop gaining MPG's. Cruise control will not account for stops and lights on your path. Thats my 2cents of advice
     
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  6. Tideland Prius

    Tideland Prius Moderator of the North
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    Those of us who have Scangauges and Gen 2s knew that by correlating the ignition timing, speed and instant mpg.

    Cruise control is meant to maintain speed. Favouring fuel efficiency requires variation in speed according to traffic and terrain.

    Holding your foot at a certain accelerator pedal angle and letting the speed vary will boost fuel efficiency.
     
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  7. bisco

    bisco cookie crumbler

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    intelligent cruise is coming, but it won't be cheap in the beginning...
     
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  8. alanclarkeau

    alanclarkeau Senior Member

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    Yes, of course you can get better doing it yourself - if you understand hypermiling.

    Cruise isn't going to get any better in a hurry.

    When they come with a Cruise Control which:
    1. can see the colour of the traffic light up front
    2. knows when the speed limit will change next
    3. knows whether you'll be going uphill or down in 50 metres
    4. can see what traffic is in the 2 lanes either side
    5. can detect the density of traffic
    6. knows what the car 3 in front is about to do
    7. knows if there's someone behind you who will get uptight if you slow over the crest of a hill
    then you can expect it to do at least as good as you. I know I can get 10% better quite easily round town - less so on the highway. But - I prefer the convenience.

    It would be different if you were driving a big Diesel truck or SUV, using 15litres/100km - 10% makes a big $$/€€/££ difference.

    But in a PRIUS, using only a miniscule amount of fuel - it becomes part of the chasing of diminishing returns - the gains sometimes aren't worth the bother - other than for bragging rights. It's great to have a bit of fun occasionally - even though it might cost a few cents.
     
  9. fuzzy1

    fuzzy1 Senior Member

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    I'm not remembering any cruise control, in any car model, that can match the fuel economy of the better drivers. Not just the hypermilers and maximalists, but many better non-hypermilers too.

    Cruise controls normally do better than the "average" driver, whatever that is. But human drivers are spread over a fairly wide distribution, and cruise controls perform not all that far above the center of the human spread.
     
  10. alanclarkeau

    alanclarkeau Senior Member

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    Agree - SWSNBN never matched my economy - but I'm sure with C/C we'd be identical - she better, me slightly worse.

    I well remember when I would do fuel returns for a fleet of vehicles - I could fairly accurately guess who had been driving a particular vehicle by the economy - or lack thereof. Could be up to 40% variance between the best and worst.
     
  11. GabrielD

    GabrielD Member

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    I am using cruise control only on highway. Drive with 87 mph for 1 hour. Fuel consumption indicated was 38 mpg (I translated metric values, wich was 140 km/h and 6.2 l/100 km).

    If anyone can repeat this test but without cruise control for comparing purposes...
     
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  12. sam spade 2

    sam spade 2 Senior Member

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    Are you also an avid user of B mode ?

    I have never seen ANY of my hybrids do what you described.
    As soon as I push the brake, it disables the cruise, engages regen and after a few seconds, the engine goes OFF.
    Having the engine rev up at unexpected times can be an early warning sign of a failing HV battery.

    Now, having said that, I do notice that my engine seems to speed up slightly sometimes when it is no longer needed to propel the car but that lasts only for a few seconds. I have always assumed that it was doing that while is senses whether or not the HV battery needs to be charged more.......or if it should just shut off. My Toyota and my Ford both do that.
     
    #12 sam spade 2, May 30, 2021
    Last edited: May 30, 2021
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  13. fuzzy1

    fuzzy1 Senior Member

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    You'd need to do your own comparison. Other people, with different tires and roads and pavement surfaces and weather and fuel blends and cargo weights and speedometer offsets, won't be sufficiently matched for comparison purposes.

    I can't do a meaningful comparison because police would try to stop me, deploying multiple sets of spike strips to flatten my tires, seriously messing up any comparisons. :mad: Maybe a few PIT maneuvers too.

    This post had a graph showing Bob Wilson's test results, but the image appears lost to a website change:
    Updated MPG vs MPH chart | PriusChat
     
    #13 fuzzy1, May 30, 2021
    Last edited: May 30, 2021
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  14. Mendel Leisk

    Mendel Leisk Senior Member

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    “Kills” (the mpg) is maybe overkill?
     
    #14 Mendel Leisk, May 30, 2021
    Last edited: May 30, 2021
  15. alanclarkeau

    alanclarkeau Senior Member

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    Yep, same here. Almost all max speeds are 100km/hr - very few with 110. And only one small area in remote Australia with 130.

    And police are $AVAG€ with fines:

    Less than 13km/h over the speed limit - $177 1 demerit point (out of 9 we start with)
    At least 13km/h but not more than 20km/h over the speed limit - $266 3 demerits point (out of 9 we start with)
    More than 20km/h but not more than 30km/h over the speed limit - $444 4 demerits point (out of 9 we start with)
    More than 30km/h but not more than 40km/h over the speed limit - $622 6 demerits point (out of 9 we start with)
    More than 40km/h over the speed limit $1,245 - 8 points and 6 month automatic suspension.
     
  16. Prodigyplace

    Prodigyplace Senior Member

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    I do not remember the Imperial Gallon equivalents, but the Gen 4 without eAWD is only rated at 52 or 54 mpg combined (city + highway) per US gallon. It appears you are getting higher than that which is pretty commonplace.
     
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