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Discussion in 'Gen 3 Prius Fuel Economy' started by Susan4ET, Jul 4, 2015.

  1. SageBrush

    SageBrush Senior Member

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    Is a grade measured as rise over run ?
     
  2. cyclopathic

    cyclopathic Senior Member

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  3. Nora

    Nora Member

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    Susan, one other thought: I only use B when the road is both steep (downhill) and icy. It makes me feel like I have a little more control. Possibly deluded about that, but I grew up downshifting when the road was slippery, then braking gingerly. I find B helps keep the speed down, with less chance of braking a little too hard. That said, with traction control and ABS, this car is not prone to skidding, but on an icy hill, I'll do anything to keep control, even if it's overkill.

    I've used B a few times on dry hills, but really just for fun. And it's not that much fun. Might as well give your foot something to do.
     
  4. Susan4ET

    Susan4ET Member

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    Let's wind up this thread...

    I got the mpg on my first tank of gas today--comes out to 45.0 mpg. Next tank should be even better because I've learned some things about using the HSI display. Depending on whether the car is already warmed up, the in-town distance, the terrain, and whether I'm using the A/C or not it is pretty easy to exceed 51 mpg in the city and very easy to exceed 48 mpg on the highway. But I have several questions and maybe some repeat earlier discussion:

    1. Would someone explain again why using "B" when dropping speed from 55 or 45 or 35 mph to a stop on level terrain hurts mpg?

    2. Depending on traffic how fast should I accelerate to 35 mph before backing off for EV or to coast, for instance? I could floor it? I could keep it off the high end of the Pwr bar? I could never let it enter Pwr? Or I could hold it back to mid way through the second half of the Econ range--but I think the later is way to slow?

    3. Likewise sort of when slowing and braking to a stop is it better to try to judge the braking needed so that the regeneration bar never exceeds or fills past full bar? Or does it not matter...that braking past a full regeneration bar still gives you even more regeneration? I suspect it is excessive and lost from the harder braking.

    4. I learned apparently that the Prius fuel computer does not use any flowmeter. That said how accurate are the mpg estimates then? In particular the one that displays briefly when you shut the car off. Depending on the trip leg conditions I have seen as good as 74 mpg down to as bad as 18 mpg so far and I have a pretty good idea why. But what are the tolerances? Is it easy to say that if this estimate read 18 or 74 mpg that it would be within +/- 2 mpg? Or better than that?

    Thanks for the help.
     
  5. SageBrush

    SageBrush Senior Member

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    So far as I am concerned, B mode does not exist.

    Just touching the Pwr bar is pretty efficient. I leave a good car length of space if not a bit more between me and the car in front when I am at a stop so that I can get a gently rolling start in EV and not have the car in front open too large a gap when the light changes to green. Not that I care -- it is a gesture of consideration for other drivers.

    If you exceed regen power the remainder of the braking is friction and becomes wasted heat. Shoot for two bars of regen or just into three.

    The real payoff for high fuel economy is not hybrid specific:
    1. Minimize braking by using coasting to slow down -- regen or not. You have to anticipate the road, the turns, and the behavior of cars around you. Leaving ample gap between you and the car in front will help a lot. The Prius turns a fuel frugal driver into a badass, but if you have wasteful habits the Prius might get you to 50 mpg.

    The meter is ~ 5% optimistic but it depends on the specific tyres
     
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  6. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    I have to say, just drive it. on your third tank, begin to figure out what you do that the MPG hates.
    First two tanks learn the controls, adjust the mirrors, find radio stations, etc.
     
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  7. SageBrush

    SageBrush Senior Member

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    This thread started out as a lame question of whether extreme use of of A/C cooling will hammer fuel economy. The obvious answer is yes.

    Moderate use of A/C cooling, however, can be a surprisingly modest hit on the fuel economy budget.
    I drive 45 miles to work and back. Over-all down to work and up back home. I usually end up at 65 - 75 mpg to work; my best is 78 mpg with some wind at my back. This week the drive was too warm for me so I turned on AUTO cooling at 78F after the car was well ventilated some 10 minutes into my drive, and finished at 76 mpg. Ambient temps approached 90F and I could not see wind so certainly the conditions were no more favorable than the day of my best result.
     
    #87 SageBrush, Jul 22, 2015
    Last edited: Jul 22, 2015
  8. Redpoint5

    Redpoint5 Senior Member

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    B mode wastes energy by converting the cars momentum into heat by revving the engine faster. It uses the engine compression to slow the vehicle down. Instead of wasting the momentum, the brake pedal can be used to regen some of the energy back into the battery.

    In a non-hybrid or EV, revving the engine would be a more efficient way to brake since it cuts the fuel flow to the cylinders. However in our hybrid, fuel is not only cut from the cylinders when slowing down, but electricity is generated and returned to the battery.

    The most efficient way would be to accelerate at 3/4 of the eco range, but the difference in efficiency is miniscule. Just drive in a way that doesn't anger everyone else.

    Your suspicions are correct. The most efficient way to slow is to estimate distance and speed to never exceed the regen capabilities of the car.

    Calculating fuel consumption doesn't require a flowmeter because other parameters are known. If you know how much fuel a nozzle of a particular size flows at a given pressure, and you know what percentage of time that nozzle is open, then you know with decent accuracy what the consumption is. I've found the Prius to be no worse than 97% accurate in estimating MPG.
     
  9. bisco

    bisco cookie crumbler

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    never even thought about using it in 11 years. but there are more mountains in neverland than around here.
     
  10. Jay705

    Jay705 New Member

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    I have a 2015 with 6500 mile my mpg 40 went to dealer and he plug in lap top and did reset on fuel computer no gettin 45 he is still working on it next Monday car should get mpg of 49
     
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  11. bisco

    bisco cookie crumbler

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    welcome, great info! please elaborate when all is said and done. all the best!(y)