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    Hank101 Member

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    I have logged 1500 miles on my Gen 3 Prius 2, and averaged 55 mpg on every tank, strictly used in my 70 mile per day highway commute. same speeds (65 mph) highway, 4 miles on and off interstate, same speed, no traffic.

    My stock tires (Yokohama S33D Avid) have been inflated to 37.5 psi, from the dealer new. Monday, I inflated all tires to 42 psi, and every day since my daily and tank average is 50mpg - no other change in commute, weather has been consistent.

    The change was immediate and very real.

    Going back to 37.5 next week for confirmation.

    Just a surprised FYI.
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    F8L Protecting Habitat & AG Lands

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    I assume you cross referenced the mpg data with local weather trends?
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    Hank101 Member

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    You assume correctly
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    F8L Protecting Habitat & AG Lands

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    Cool. It will be interesting to see how this test pans out. The only explanation I can think of is your road surface may not be smooth enough to gain anything from higher pressure and it causes the harder tires to bounce and skitter.

    WalterLee runs the same tires and found his best mpg comes from 50psi I believe.
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    ngc4565 Member

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    This is very interesting, but it makes no scientific sense whatsoever. Please keep us posted with more data.
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    Hank101 Member

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    I can not explain why this happened, and search for verification of truth. But again, it was real, and immediate.

    I previously owned a 2009 Jetta TDI, and ran max sidewall pressure 50 psi on my Michelin Energy savers. +2 mpg, but needed all of my dental fillings replaced afterward.

    My first thoughts are slight tread pattern change, possibly requiring additional break-in. I wonder if there is a TPMS signal influencing something???
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    SageBrush Senior Member

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    Check your tyre pressures again.
    If they are still at 50 psi then I think the association is coincidental, and a search for the actual cause of the MPG drop is in order.
    lol
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    MrBillTulsa Member

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    Could there be a measurable change in tire revs per mile with the different tire pressures?

    :eek:
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    friendly_jacek Senior Member

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    Shouldn't be measurable.
    Makes no sense.
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    walter Lee Hypermiling Padawan

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    Very interesting. Thanks for the posting. :D

    However I like to point out, your front tire pressure is suppose to be two pounds less than your rear tire pressure. For example, if your front tires are set to 42 psi then your rear tires are set to 40 psi. This is because most of the weight of the car is in the front - the lower rear axle tire pressure increases driving traction on the rear tire tires and hence provides better emergency handling. So if if the front tires are set to 37.5 psi then your rear tires should be set to 35.5 psi.

    Your safety is more important than any incremental fuel efficiency gain.

    Are you using Cruise Control(cc)? If you manually attempting to maintain 65mph what is your highway speed range?

    Overinflating your tires reduces your tire's rolling resistance and hence your driving traction (which is not the same as braking traction). The best way to increase the Prius's fuel efficiency due to a tire's lower rolling resistance is to drive on very smooth dry asphalt roads when wind resistance is not a major factor (speeds under 40 mph) and attempt extended the time a Prius *glide* (move on momentum alone without the electric motor or the ICE running) -- if the road surface is not dry (e.g. if it is raining ) then there is loss of driving traction which makes it harder to accelerate at a high rate. I found that with over inflated tires, especially when the road is not dry, the Prius' energy efficiency during an acceleration cycle decreases as the the acceleration rate increases. In layman's term, the more you over inflate your tires, the slower your accelerate rate must be to. If the road is rough (e.g. grooved before construction, rumble strips, potholes everywhere) then over inflated tires can reduce your fuel efficiency and give you a sore body afterwards too. If you over inflate your tires - your FE will drop as your acceleration rate increases and as your speed increases over 40mph. Tire overinflation can help the Prius achieve +60mpg but only if the Prius top speed is between 25 mph and 40 mph, the acceleration rate is very slow, and the road surface is smooth and dry. For example, when hypermiler wayne gerdes test drove the Prius C on the California superhighway - he drove it with the tires inflated at Toyota's recommended pressures as defined by the driver's door jam sticker (that is he didnot over inflat the tires) but when he was doing a 20mph-course MPG marathon with the 2010 Prius he overinflated his tires 10 pounds over the tire's maximum sidewall pressure setting!


    In general, the energy efficiency of low rolling resistant tires drops when road surfaces are poor (rough or wet) OR when driving speeds increase over 45 mph.

    At speeds greater than 38 mph wind resistance starts to increasing take more energy to overcome and by 65 mph wind resistance is taking over 50% of the Prius' energy cost and is forcing the Prius to constantly expend energy/accelerate to compensate for the wind resistance. As you found out as the Prius speed increases over 55 mph the advantage of overinflating the tire pressure is not as important and can actually hurt the Prius' FE. With over inflated tires you need to accelerate more slowly to even achieve the same FE level of the lower tire pressure settings because your driving traction is reduced. This energy efficiency paradigm is comparable to an airship vs an airplane: An airship is more energy efficient than an airplane at low speed but at higher speeds an airplane can be more energy efficient than an airship. A hybrid's major energy efficient advantage is for city driving hence their City MPG are very high. Most manufacturers of conventional gas car (e.g. Chevy Equionox) only post their highway MPG because that when an ICE is most efficient. If your Prius is mainly going at highway speed what's more important is mitigating wind resistance: which include rolling up your windows, eliminating external wind drag resistant items (e.g. roof/ski racks), reducing your top speed, driving behind another vehicle slipstream(aka drafting, which can be dangerous if not coordinated with the lead vehicle), driving with a tailwinds, and slowly accelerating to your top speed before setting the CC. Note that CC is not the most fuel efficent way to drive at high speeds -- a Hypermiling technique called Driving with Load (DWL) delivers the better fuel efficiency - but it takes skill and hard work ( I can only do it for about 1 hour before my body just gives out) and the road's traffic condition has to allow for it.



    The easiest way to increase your fuel efficiency on a Prius...
    ================================

    If the highway is dry and driving temperatures greater than 60F, then set your Prius to Cruise Control to 60 mph. Under those conditions, the Prius should be able to get between 58 mpg to 64 mpg (YMMV depending on such as wind conditions, road surface types, overall road elevation changes
    [*]).

    HTH

    Walter Lee
    aka 'HyperDrive 1' on CleanMPG, An authoritative source on fuel economy and hypermiling mileage logs
    2010 Toyota Prius III , Blue Ribbon/ Dark Grey, OeM floormats
    Yokohama Avid S33D ( front 50psi rear 48 psi)
    ScangaugeII (AVG,SoC,RPM,GPH)+(FwT,TPS,IGN,iMPG)
    Grill Blocking ( on and off depending on the driving temperatures)
    odometer= +20400 miles, overall 60.9 mpg
    MfD Average speed = 18 mph (median top speed 35 mph)
    [*] YMMV(Your mileage may vary): Tail winds (winds pushing from behind the car) increase your MPG. Head winds(winds pushing againt your car in in front of it) and Cross winds(winds pushing to the side of your car) will reduce your MPG. Rough or wet road surfaces (e.g. road construction) reduces your MPG. Smooth dry road surfaces can increase your MPG. A drop in overall road elevation ( you are going downhill) can increase your MPG. An increase in overall road elevation ( you are going uphill ) can decrease your MPG. Also getting stuck in a traffic jam doesn't help either. :doh:
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    friendly_jacek Senior Member

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    I think most would disagree with your reasoning here.
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    bisco cookie crumbler

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    some things in life are counter intuitive. such as accelleration rate and mpg's. until someone does true controlled testing, we'll never know for sure.
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    ghosteh Member

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    It's not worth overinflating & reducing your tires life by 10,000 miles just to gain a few mpg.

    I love playing the max mpg game too, but there are some limits. I like cruise control, air conditioning, staying within the reasonable flow of traffic (not being a rolling roadblock), and don't want to wear my tires out prematurely.
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    F8L Protecting Habitat & AG Lands

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    Where did you come up with that figure? Most people who overinflate their tires on the Prius, and many other cars, do not suffer from reduced tread life. The tires wear very evenly. In fact, on the old GenII with Integritys people who didn't overinflate were the ones who wore their tires out prematurely. :)

    Seriously though, most of us watch tire wear pretty carefully and in most cases the tread wear is very even.
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    walter Lee Hypermiling Padawan

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    I know it is very counter intutive. I am hoping someone can prove me wrong because I'd rather have one tire setting for both city and highway driving. My real world experience is based only on with overinflated Yokohama Avid S33 tires - another tire make and model may have different characteristics - but I have a feeling that what I am experiencing is a general scheme of things, i.e. as wind resistance becomes a major forceto overcome - a tire's rolling resistance becomes less important and a tire's road surface traction becomes more important. All-Season tire's design like my Yokohama Avid S33 is a compromised performance over a broad range of driving environments/situations.

    When a car is gliding(moving on momentum only) then a vehicle's low rolling resistance is more important to fuel efficiency but when a car is pulsing (accelerating) then a tire's ability to transfer a tire's energy into horizontal motion by pushing against the road surface is important to fuel efficiency.

    When you are driving under 45 mph you can glide a longer distance without apply the accelerator so low rolling resistance tends to give better fuel efficiency. However, when you are driving over 45 mph you have to apply the accelerator for a longer time which means that a tire's ability to grip the road and efficiently transfer motion from the tires is more important. With over inflated tires, I notice I'm only getting the high fuel mileage when I'm driving at low speeds where wind resistance is not much of a factor and the road surfaces are dry and smooth. When I've test the Prius several time on 10 hour superhighway trips (50mph to 75mph) using two tire over inflation setting (40/38, 44/42) and cc, the MPG results were in the mid 50s - which are dismally bad for a hypermiler. What really improve my highway FE was when I either drop my top speed below 56 mph or when I used DWL.
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    walter Lee Hypermiling Padawan

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    I'm in my 23rd month of a 24 month hypermiling trial experiment where I've been teaching myself how to maximize the fuel efficiency of a 2010 Prius. I've gradually over inflated my Yokohama Avid S33D - because I wanted to test out different settings. Over the last 20400 miles, I haven't reduce my tire tread by very much. I think one of the reason is that my tire tread looks as good as it does is because I am driving at low speeds(avg speed 18mph), I am accelerating gently, and I slow down and brake gently as well, i.e. my driving habits is not likely to burn any rubber. :D Not everyone can adapt my driving pattern and I am not expecting them to - but those who could and do should see similar results.

    Overinflating tires by itself does not increase fuel efficiency but it amplifies/increases the results of low speed fuel efficiency driving techniques like Pulse and Glide. If the driver doesn't know how to drive fuel efficiently then overinflating the tires will only increase a vehicle's MPG by a very minor amount.:rolleyes:
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    sidecar Member

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    assume the contact patch has changed and the tyres are bedding in again
    IOW he needs a longer time sample
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    elcano Junior Member

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    Getting results that go against conventional wisdom is OK (and sometimes really important), it just needs to be reproducible.

    While I'm no expert in statistical design of experiments, I would try to repeat the experiment about 10 times with each pressure level (20 weeks or tanks in total) and perform a t-test to check if the difference is significant. If you run another 10 experiments while underinflated (for the sake of science) it should be possible to perform a nice linear regression and chart.
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    ghosteh Member

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    I pretty much pulled it out of my arse. :p I didn't mean for it to be a factual comparison, but just threw it out as an example.

    Isn't the whole idea behind tire pressure that you're reducing rolling resistance by "ballooning" the tire and making it more rounded in the contact area? Lower pressure would mean the tire would spread out and have a wider contact patch, so isn't the opposite true?
    And if that's the case, doesn't overinflation lead to premature & uneven wear?
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    F8L Protecting Habitat & AG Lands

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    LOL :D
    Sort of but the larger reason is that the higher pressure stiffens up the sidewall/plys and reduces internal deformation (hysteresis). A properly overinflated tire should not reduce the contact patch by an appreciable amount.

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