Effect of tire pressure on mpg: collecting data

Discussion in 'Gen 3 Prius Fuel Economy' started by Robert Holt, Sep 29, 2019.

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  1. ice9

    ice9 Active Member

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    Projecting your views as those of another, eh?

    "Cognitive bias" was the point... ...Maybe we should revisit that subject.:rolleyes:

    Also the "anecdotal": Just because it is used, doesn't necessarily mean it's incorrect - though it might be, in this case... ...or it might not. Over-pressure is often thought to improve mpg simply because it's opposite is known to reduce mpg. So the most you can say about Ray's comment is that it might be a little misguided.

    But that certainly doesn't make it biased. Calling Ray's observation a cognitive bias is like calling an optical illusion by the same name - the difference being that of a psychological predisposition due to personal bias, as opposed to those natural/organic cognitive predispositions that we all have in common. If anything, Ray's comment would fit the later definition, not the former...

    ...But then, there is also the possibility that he could be correct. My understanding is that the jury is still out regarding the over-pressure debate, mainly because there are too many variable to make a positive assertion either way.
     
    #21 ice9, Oct 12, 2019
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  2. ice9

    ice9 Active Member

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    #22 ice9, Oct 12, 2019
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  3. William Redoubt

    William Redoubt Senior Member

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    Cognitive bias is why optical illusions work. We cognitively expect a different result.

    The only thing I know is that I get much better mileage at 40 psi with my Hankooks. There is no doubt about this. I have measured the difference.
     
    #23 William Redoubt, Oct 12, 2019
  4. Grit

    Grit Senior Member

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    I would, but then imagine all the feedback I'll get on here for not putting wipers, back seats and everything back in to make it more the average every day prius driver. Then I'll have to drive without the music on, gas up before every commute, note daily wind direction, buy new tires because my tread height are 3/32", find a circle track and alternate between drive directions. After responding to replies / feedback, I'll end up abusing the "ignore" button feature ha.
     
    #24 Grit, Oct 12, 2019
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  5. Robert Holt

    Robert Holt Senior Member

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    Hey Grit, why not put in your data? I believe you mentioned your tires were leaking down in air pressure and that you re-inflate them on a regular, predictable basis. How often do you re-inflate them? Say you do that once a week on Sunday: If you zeroed out Trip B right after you re-inflate to 40+ psi and record the mpg for the Monday-Wednesday period you get the “high pressure” data point. Then zero out Trip B again and record the mpg for Thursday-Saturday. When you couple that result with the measured extent of lower tire pressure right before you re-inflate (say 36 psi), and you have the “low pressure “ data point. I’d be interested what differences you are finding on your normal driving routine. Cheers!
     
    #25 Robert Holt, Oct 12, 2019
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  6. William Redoubt

    William Redoubt Senior Member

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    I don't think that 40 psi is over inflated. My observations show this. Why do you think that 35 psi is the proper inflation for Prius tires?
     
    #26 William Redoubt, Oct 12, 2019
  7. Robert Holt

    Robert Holt Senior Member

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    Another paired data point for our standard Fairfax-Manassas shopping loop (winds light and variable, temps 53-60 Fahrenheit, no A/C):

    Tire pressures at 42 psi all around:
    B48E2D5D-3CFC-4B20-B474-C98FCA04134C.jpeg

    Tire pressures set at 34 psi all around:
    62AEAC1F-6FE4-46EE-BC47-2B82866F3FFC.jpeg

    So the observed difference is 5.4 mpg or a roughly 8% decrease in mpg due to the lower pressures. Since this pair of data points had an 8 psi change in the cold tire pressures, the average percent change in mpg per unit change in psi is 1% , which is larger than expected.
    (Auxiliary Note: She Who Must Be Obeyed noticed a subjective change in the ride, with the higher tire pressures associated with a rougher ride.)
     

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    #27 Robert Holt, Nov 11, 2019
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