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Interesing PSI MPG Observation

Discussion in 'Gen 3 Prius Fuel Economy' started by Hank101, Mar 22, 2012.

  1. walter Lee

    walter Lee Hypermiling Padawan

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    F8L is correct.

    However, I would like to further clarify what it means to have tires that are properly overinflated. A tire's tread surface which is in contact with the road surface does not substantially change between the car's manufacturer's recommended tire pressure setting that is listed on the driver's door frame and the tire's maximum pressure (cold) setting that is listed on the tire's sidewall. What seems to change within this pressure range is the vehicle's handling characteristics. At higher tire pressures still under the max sidewall tire pressure - mechanical shocks from road surface irregularities are transferred more efficiently and lateral road traction to resist cross winds is somewhat weaken. Acceleration at speeds higher than 45 mph where wind resistance is more of a factor is also less efficient. However, the tire has lower rolling resistance and can coast further at speeds lower than 38 mph. The tire's surface contact patch decreases as the tire pressure goes over the maximum sidewall tire pressure, e.g. for a Yokohama Avid S33D the maximum sidewall tire pressure is 44 psi so has the pressure goes over 44psi there starts to be a decrease in tire tread road surface contact. The loss of road surface to increase air pressure is gradual and the road traction and braking traction loss are gradually felt as the speed of the vehicle increases or when road surface traction are lessen by rain, ice, or snow. Depending on the driving temperature, the length of time the tire is being used, and the speed of the tire - the tire set at a very high cold tire pressure setting could have different traction/driving properties if the tire pressure increases due to higher operating temperatures.
     
  2. Former Member 68813

    Former Member 68813 Senior Member

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    I would love to see any evidence that higher tire pressure decreases acceleration.
     
  3. F8L

    F8L Protecting Habitat & AG Lands

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    In a drag racing application, higher air pressure tends to increase mph at the end of the 1/4mile. This is most likely due to reduced rolling resistance.
     
  4. SageBrush

    SageBrush Senior Member

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    My original reason to pump up the pressure on my G2 Prius tyres was to improve longevity. At 'spec' pressures the tyres were not wearing evenly, leading to early replacement under warranty by 1/2 the rated miles they were supposed to last. The car now has a different tyre (Michelin Hydroedge) so I cannot compare apples to apples, but at max sidewall pressure they will exceed the manufacturer distance rating.

    Lower pressures allow a less bumpy ride, but higher pressures improve fuel economy, longevity, and stopping distance. I think high(er) pressure is an easy choice for people who drive on decently maintained roads.
     
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  5. Judgeless

    Judgeless Senior Member

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    I have a set of snow and summer times.

    I have been running 49 PSI in the front and 48 PSI in the rear since Oct 2009 in my 2010 Prius. I have close to 80,000 miles and the tires are showing even wear and I predict I will get another 80,000 out of them.
     
  6. revhigh

    revhigh MPG Enthusiast

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    This is an interesting discussion, but I'm not sure how you can make the above MPH statements with any scientifically backed degree of certainty.

    Particularly the SECOND statement in the quote.

    You say that the tire has a lower rolling resistance with higher PSI ... of course that's true, but how you come up with a number like 38 MPH baffles me.

    If a tire has a lower rolling resistance at 30 MPH, it also has a lower rolling resistance at 60 MPH ... compared to a tire that is inflated to a lesser PSI. Granted, the degree of rolling resistance reduction may be slightly less due to air resistance at higher speeds, but to say that it has a LRR at 38, but not at 60 makes no logical sense.

    In other words ... let's look at it this way.

    Let's say you're travelling at 30 MPH and then release the gas pedal and you coast 1000 feet. Then you travel at 60 MPH and you coast 1900 feet. You could deduce from that, that with higher speed you do not coast as far as at a lower speed, because your speed doubled, yet your coasting distance did not. This would possibly be true. However ... this doesn't mean that your tires have lost some of their RR because of speed, it just means that air resistance did not allow as much coasting.

    It seems that you're saying that lower PSI's yield higher MPG's at highway speeds ... and I simply don't buy that at all.

    Given the example above, with 1900 feet coasted starting at 60 MPH ... I would say that a DECREASE in PSI's would yield a commensurate DECREASE of MPG's (coasting distance) .... every single time ... it would NEVER result in an INCREASE in RR (or distance coasted) with all conditions being kept equal .... at ANY speed.

    REV
     
  7. revhigh

    revhigh MPG Enthusiast

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    I'd think it's ALSO due to increased diameter of the tires (yielding more feet travelled per revolution of the tire) due to a given amount of air being heated. More air (higher PSI's) ... to start ... yields more diameter due to air expansion of that much more air ... at the end of the run. :D

    We've ALL seen the way drag racing tires 'get taller' during a burn out, which is probably due to air pressure increases due to heat, as well as centrifugal pressure.

    REV
     
  8. walter Lee

    walter Lee Hypermiling Padawan

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    Premise:
    Your tire air pressure setting can change a tire's performance where there is a trade off between rolling resistance and road surface slippage.

    My methodology:
    I've driven several +600 mile road trips testing out overinflated tire pressures.
    I started out with toyota recommended tire pressure setting and I have been incrementally increasing my tire pressure up to study the effects of higher tire pressure settings. Currently I am using a tire pressure setting of 50psi front/48 psi rear on Yokohama Avid S33D ( max sidewall is 44 psi).


    My Observations:
    I've been using overinflated tire settings since May 2010 and I've experienced very low and very high MPG results (using a ScangaugeII) with overinflated tire setting. Under inflated tire pressure setting will always decrease your fuel efficiency. My experience has been that overinflated tire pressure setting can in certain situations increase your fuel efficiency, BUT that overinflating a tire's air pressures is not a silver bullet and will not always help you increase your fuel efficiency.

    Increasing a tire air pressure will decrease rolling resistance but it can also increase tire slippage. An increase in tire road surface slippage (decrease in road surface traction) will decrease a tire's energy efficiency during acceleration. When a tire is used only to conserve momentum(maintain a velocity) then low rolling resistance can be help. When a tire is used more frequently to accelerate and to overcome forces like aerodynamic drag then tire traction is necessary. As the Prius goes over 50 mph, the Prius must work harder to constantly over come aerodynamic drag - hence tire pressures closer to toyota's recommend pressure setting is likely to produce less tire slippage and more efficient accelerations. However, when the forces of aerodynamic drag are minimal then (<=38mph), higher tire pressures that lower a tire's rolling resistance can help the Prius achieve higher than normal fuel efficiency.

    Science:
    When a car is driven the air inside the tire will heat up. The air inside a tire will heat up faster and hotter the faster and longer a tire is driven. Via the Ideal Gas Law, at the end of a road trip the tire air pressure will be higher after being driven at 60 mph (because the air will be hotter) than after being driven at 30 mph given the same trip time period(because the air should be cooler).

    Lower tire rolling resistance(road surface traction) works to maintain a vehicle's momentum and velocity by minimizing rolling drag. As long as gravitational forces and drag forces do not decrease the velocity down - your car will continue to go forward in the same direction and your MPG continues to go up. When the car must accelerate and overcome inertial load and aerodynamic drag, then the tire's road surface traction in very important to avoid any tire-to-road surface slippage while it push the car forward. When the road is wet/slippery because of rain, ice, snow, debries, or is just even or rough - then tire-road surface slippage will reduce the mechanical efficiency of the tires to move the car forward. Even under perfect dry smooth road condition there is some tire-to-road surface slippage but as long as this is kept minimal and drag/acceleration forces required are very low THEN the amount of tire traction necessary is also very low SO we can afford to set the tire pressure for a very low tire rolling resistance ... This is more often the case when acceleration is done gently and slowly. When acceleration is done more vigorously this is not the case THEN tire pressure closer to toyota's recommend tire pressure setting wold probably offer higher tire traction and thus achieve better results in both handling an fuel efficiency.

    In Practice/Hypermiling results
    In an optimum pulse and glide cycle, the pulsing is short cycle and the glide is an extended cycle. Hence with higher tire pressures, Even if a pulse session of a tire might be less efficient -for a short time- a longer more efficient glide cycle could more than makes up for it. As the glide cycles become shorter and shorter than its relative advantage become less and thus the any tire-road surface road slippage energy transfer inefficiency becomes more apparent. As a vehicle's speed increases over 45 mph aerodynamic drag reduces the glide velocity at a faster rate and as te speed range of the Pulse and glide narrows then the glide cycle becomes shorter relative to the pulse cycle and hence the Pulse and Glide less and less useful at higher speeds. That is to say if you are doing a PnG at above 50mph - in order to get an energy efficiency gain one might need to atleast have a 12mph range and that a 6mph range would not likely provide any FE gain at higher tire pressures. That is to say - for speeds greater than 65mph using a narrower range of speed (64mph to 70mph) that have a shorter pulse and glide session a lower tire pressure setting of 38/35 psi with more rolling resistance/tire road surface traction might be better than a higher tire pressure setting of 44/42psi. Because such a wide range difference in speeds is not practical on the superhighway - most hypermiler normally focus on conserving a vehicle's gravitational energy via Driving with load (DWL) sessions rather than focus on PnG sessions.

    hope this helps

    Walter Lee
    "Hyperdrive1" at cleanmpg.com
    2010 Toyota Prius III blue Ribbon/Dk grey



    in related topics

    Farming Tractor tire traction vs tire pressure

    http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/eng8081/$file/Effect_of_Correct_Pressure_on_Tractive_Efficiency_of_Radial_Ply_Tires.pdf?OpenElement

    http://www.nationalwatermelonassociation.com/pdfs/news/Reduce%20Compaction%20And%20Tire%20Slippage%20In%20The%20Field.pdf

    Racers discuss tire slippage during turning

    Tire Technology, excerpt no. 2 from The Racing & High-Performance Tire
     
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  9. Former Member 68813

    Former Member 68813 Senior Member

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    I don't how there could be even minimal tire slippage without snow, ice, mud, or gravel. Most people drive on maintained asphalt or concrete roads.
     
  10. walter Lee

    walter Lee Hypermiling Padawan

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    Warning: Heavy Science stuff...

    All energy transfers done by frictional forces, e.g. CVT transmissions, have some slippage loss. Due to the laws of thermodynamics (entropy) all energy transfer systems are lossy - there is no perpetual motion machine. Snow, ice, mud, and gravel can cause major tire slippage; however, when looking at tire slippage from a fuel efficiency focus - we are looking at fractional energy losses that over time can affect fuel efficiency of a vehicle. For something you can see, smell, hear and feel --- Tire slippage loss is what Racing folks and other gearheads called "burning rubber." The easiest way to see this is in the form of a drag racer starting to accelerate from a dead stop; initially the motor drives the tires causing the tires to rotate at a very high speed. The tire treads slips as it rotates against the road surface creating frictional loss in the form of heat - as the heat builds up rubber will burn off the tire creating hot smoke and the smell of burning rubber as it suddenly accelerates from the starting line. Back in the normal world of every day driving, the tire slippage and frictional losses also cause your tires to heat up over time as well but you have to look really closely to see how you are losing this energy. In the real world most of this energy loss escapes as heat. For example, if you drive your car over 60 minutes at over 65 mph you find that the air in your tires will heat up several psi because the air in your tires are warmer than than when you start your trip. As the air in your tire heats while in a fixed volume the pressure of your tires will increase (this is called the Ideal Gas Law). You'll also notice that if you accelerate very gently and drive less than 35 mph that the air pressure in your tires will not increase as much (because you have lost less energy via tire slippage so you have created less frictional heat loss). That is to say ...to maximize fuel efficiency gains using overinflated tires a driver must reduce tire surface slippage by accelerating gently and limiting his/her top speed.


    hope this helps

    Walter Lee
    HyperDrive1 @ Cleanmpg.com
    2010 Toyota Prius III Blue Ribbon/Dark Grey,Oem Floormats
    Yokohama Avid S33D (50/48 psi)
    Grill blocking (on and off - with this crazy weather...)
    Scangauge II (AVG, Soc, GPH, RPM)(VLT, Fwt, IGN, Tps)
    Odometer =+20600, 60.9 mpg, avg 18 mph, 10% ethanol 87 oct gas
    16 mile/55 min morning commute (300 ft elevation drop)
    16 mile/60 min afternoon commute (300 ft elevation rise)
    best tank: 727 mile/ 10.25 gallons/ 70.9 mpg
     
  11. jhinsc

    jhinsc Senior Member

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    I understand the thought behind increased tire pressures for longer tire life and more mpg's, but I also factor in ride comfort. I tried upping mine to 42lbs front and 40 rear but couldn't take the brittle ride characteristics and also tended to wander down the road and having to constantly do minor steering corrections. The highest I run mine is 39lbs front, and 36 rear. This setting gives me the best combination of ride and mpg. Last tank was 54mpg calculated and recently averaging 52+ over several fillups. My driving style is pretty normal, going with flow of traffic, always in the red/power zone when accelerating from stop signs/lights, and don't pay too much attention to hypermiling except for slowing-coasting when I can and not exceeding speed limits by more than 10mph. My commute is 23 miles each way, with 19 stop signs or lights, and speed limits of 35 - 55 mph.
     
  12. Former Member 68813

    Former Member 68813 Senior Member

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    No, there is no science in what you said. You are confusing tire slipping with energy transfer loss and you incorrectly attribute rolling resistance (that heats the tire) to tire slipping.

    BTW, good luck with "burning rubber" in Prius.
     
  13. walter Lee

    walter Lee Hypermiling Padawan

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    Overinflating tire decreases the tire's tread-to-road-surface contact area and makes the tire more ridge (so it doesnot deform) and this can cause the tires to slip or bounce( lessening a tire's tread-to-road-surface traction).

    When the Prius goes over 55 mph, that the fuel efficiency gains from lower rolling resistance (created by over inflating the tires over toyota's recommended tire pressure setting) can be cancelled out by a decrease in road surface traction and thus less efficient accelerations. I have also noticed (like the OP) in my own highway tests (speed range 55mph to 80 mph) that overinflated tires (40/38, 44/42) don't appear deliver significantly better fuel efficiency at high speed (avg 58 mpg ). Strangely, over inflated tires can help achieve more significant fuel efficency results at lower speeds (25mph to 40 mph). This is because at low speeds, the Prius does not frequently use the accelerator. However, in highway speeds, the accelerator is almost alway on and the ICE must work constantly to overcome wind resistance.

    I believe that as the Prius tire air pressure increases over its recommended setting (35/33psi) that the tire traction will starts to degrade (with tire slippage is becomes more significant) and that this decrease in tire traction will reduces the Prius fuel efficency as the Prius velocity goes pass 55 mph.
     
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  14. F8L

    F8L Protecting Habitat & AG Lands

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  15. Former Member 68813

    Former Member 68813 Senior Member

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    That is a well know fact that as speed increases, air resistance increases too and relative contribution of tire resistance decreases (even though the absolute increases some).

    I didn't see anything in the study that addressed the issue Walter champions that supposedly higher tire pressure decreases traction and hence increases rolling resistance at higher speeds.
     
  16. Flying White Dutchman

    Flying White Dutchman Senior Member

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    mm this is intresting
    i am normally at 4 bar but last time i visited the dealer i asked them not to decrease it and they did not because i told them i always do this and its better MPG
    but they even put more in it
    after that i got a MPG hit that i was thinking because my big service done like spark plugs and oil
    but now that some time ago and still not that good....
    i will go to the gas station soon and get it back down to 4 bar .. what is still hill but lower then they did and see what happens
     
  17. F8L

    F8L Protecting Habitat & AG Lands

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    Correct, it doesn't address that opinion. I am not ready to subscribe to the idea either. However, I don't think Walter is saying rolling resistance increases as speed increases. I think he is saying that there may be higher amount of tire slippage which steals energy that should be used to propel the car and thus lowers total efficiency. This has been addressed in other documents I have read, but currently cannot recall, although the main concern was with tires bouncing over uneven surfaces when overinflated as opposed to flexing and conforming to surface imperfections.
     
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  18. AtoyotA

    AtoyotA Junior Member

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    Wow, interesting reference. Ref. 5 in the above document is to a DOT publication on "The Pneumatic Tire". (707 pgs!). Chapter 12 of this tire document talks about rolling resistance. They describe it thusly;
     
    Rolling resistance includes mechanical energy losses due to aerodynamic drag associated with rolling, friction between the tire and road and between the tire and rim, and energy losses taking place within the structure of the tire. Bearing losses at the tire spindle, however, are excluded, since they result in a reduction in the energy transmitted to the tire.

    My point being, friction (aka slippage) between the tire and road is recognized as a component of rolling resistance, which is what WalterLee is talking about.
     
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  19. walter Lee

    walter Lee Hypermiling Padawan

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    I understand the disbelief - I was hoping that decreasing
    the rolling resistance would also help increase my MPGs
    at speeds over 55 mph too. But - my experience and the
    repeated results from my scangaugeII says otherwise.
    Maybe my scientific hypothesis need refining - but my
    results have been undeniable and consistent. My
    highway fuel efficiency experimenting results with tire
    overinflation has been consistantly disappointing over
    several tire overinflation settings.

    Rolling resistance decreases (but not linearly) as the tire
    pressure is increases but as the air pressure increases so does
    tread-road surface traction decreases. At speed over 55 mph
    when you are pressing the accelerator almost constantly, the
    Prius starts to needs higher tread-road surface traction more
    than it needs lower rolling resistance to get the best fuel
    efficiency because the tires must start to push more often
    against the road at those higher speeds. Tire slippage is more
    of a problem with tire overinflation because the tires
    become more ridge/stiff and donot deform as easily -hence
    any road surface imperfections will tend to push the
    tires up as the tire strikes the surface imperfections
    rather than deform to those road surface imperfection
    and allow the tires to grip the road surfaces while
    it is pushing the Prius forward against the road. When
    a car is coasting/gliding - the decrease in tire road surface
    traction is not going to affect the car's fuel efficiency
    but I believe that when the car is pulsing (the accelerator
    is being applied) then this weaker tire road surface
    traction leads to a less energy efficient scenario.
    This is not to say we dont need low rolling resistance
    tires but that at certain high speeds low rolling resistance
    becomes less a factor than tire road surface traction.

    That is too much of a good thing might be bad for you
    or More might not always be better.
     
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  20. NortTexSalv04Prius

    NortTexSalv04Prius Active Member

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    Walter Lee
    What is your current tire psi?(front/back) Additionally what is your current mpg? Do you use SG2?