Inflate tires to MAX...Lose 5-7 mpg???

Discussion in 'Gen 3 Prius Fuel Economy' started by Phish0742, Nov 1, 2011.

  1. Phish0742

    Phish0742 New Member

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    My 2011 has had the original tire pressure since I bought it in July. I checked it the other day and it was about 10lbs below the MAX on the tire. I filled it up to the max and now I lost 5-7 mpg. This cannot be normal. Nothing else has changed on my routes that I have take so the other variable I can see is the tire pressure. Anyone seen this?
     
  2. Codyroo

    Codyroo Senior Member

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    Has the weather changed? (gotten colder, windier?)
    What else has changed since you inflated your tires to the max?
    You could release the extra air and see if you mileage returns (I doubt that would happen)
     
  3. Phish0742

    Phish0742 New Member

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    The temp has gone down recently, but in the 2 weeks after I inflated them, the temp was still about the exact same as it was when I was getting great mileage.
     
  4. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    has nothing to do with tire pressure (although not putting the extra air in reduced your performance loss) its the weather
     
  5. fuzzy1

    fuzzy1 Senior Member

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    Winter fuel blend should be here, which always hurts. Not being in your geography, I cannot verify your claim of "the temp was still about the exact same" or know whether precipitation increased -- before your weekend snow dump, with should have really hurt MPG.

    My area's temperatures have already dropped sharply, and my MPG is down almost 5 mpg from the summer high. The online graph isn't updated yet, but look here to see how much my MPG fell the past two fall/winter seasons.

    See also Why mileage gets worse in winter
     
  6. F8L

    F8L Protecting Habitat & AG Lands

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    Like the others have stated, it has nothing to do with your tire pressure. There are a lot of other variables that could cause the loss. Namely weather, wind, short trips that you forgot you made etc..

    Be sure to read the link fuzzy posted.
     
  7. bisco

    bisco cookie crumbler

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    never make changes you hope will effect mpg's when the season's are changing. and they change pretty darn rapidly around here. my guess is the increase in tire pressure was negligible and the winter blend gas was dramatic. i just got hammered in the last week or two.
     
  8. alfon

    alfon Senior Member

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    When my wife drives she almost always gets 5 mpg less than I do, even though all 4 tires are filled up with Nitrogen to max sidewall pressure of 44 PSI.

    The tires, Michelin Energy Saver 195x65x15 have 50,000 miles on them and they are wearing even and quite well with an estimated 15,000 miles tread wear left...

    Some people have stated that the inside tread of the tire will wear quicker than the outside tread with Max sidewall pressure. This is not the case with these tires.
     
  9. walter Lee

    walter Lee Hypermiling Padawan

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    Setting your tire pressure to the Max sidewall only helps increase fuel efficiency if several other conditions follows:

    (1) The road surface is smooth (black asphalt is the best) and dry.
    If the road is being resurfaced/grooved or is in bad shape then
    a tire which is too rigid will cause the tire to bounce and lose road
    traction/fuel efficiency - hence if your road is being resurfaced/grooved
    or is in bad shape you need to lower the tire pressure back to 35/33psi.
    If the road is wet, icy, snowy, or just has alot of mess on it then
    having tire pressure greater than 40/38 psi will not likely
    provide better fuel efficiency because when the road surface
    is slippery, icy, snowy, or messy - more road traction is necessary
    to get the same fuel efficiency so you can't have the tire being
    too rigid (the tire surface needs to be able to deform around the
    road surface irregularities to get a better grip on the road).
    BTW the faster the tire travels - the more often this is true. If
    the tire travels under 28 mph then the loss of road traction due to
    road surface irregularities has a much smaller effect on fuel
    efficiency than if that same tire is going at 56 mph. The higher the
    speed the more signficant the smoothness and dryness of the road
    surface matters with respect to FE when using over inflated tires.



    (2) The vehicle speed need to be kept under 60 mph and mainly between
    27 mph to 46 mph to achieve the Prius's optimun fuel efficiency.
    If you race the Prius you can forget about FE.

    (3) The driver coasts/glides to a stop (terminal velocity should be <15mph)
    and gradually accelerates( minimize top speed for uphill accelerations)

    (4) The driver minimizes stop and go driving

    (5) The outside temperature is stable, the optimum temperature 65F to 90F
    (6) Stay away from E15 gasoline.


    About this time in your region (Massachusetts) you are going to have colder weather and Reformulated gasoline (especially in urban areas) this will cause your MPG to drop by 5-7 mpg. I'm in the Washington DC - south of you - my mileage has dropped by about 10 mpg since October 20,2011. From July to September I was getting from 65mpg to 83 mpg (average 70 mpg) on a 16 mile commute; However, after October 20th 2011, this same 16 mile commute - my 2010 Prius fuel efficieny has dropped to 55mpg to 69 mpg (average 60 mpg). I've got my tire pressure set to 50 psi front and 48 psi rear, both top and bottom front grills are blocked, the 16 mile commuting route is designed to optimize hypermiling results, and I'm hypermiling using a ScangaugeII. The MPG loss in the Prius due to temperature is less the longer the Prius is driven. If you drive less than 30 minutes and 5 miles per trip than you should see a big drop in you MPG. If you drive for over 60 minutes and 10 miles per trip you should see a smaller drop in MPG. If you are driving over 15 miles /2 hours or more in one trip (with some superhighway driving) the drop in MPG should be very small. One drastic solution for keeping the Prius MPG high on the Cleanmpg.com mileage log was not to drive the Prius during the Winter! LOL - but where's the challenge to that? :-P
     
  10. sipnfuel

    sipnfuel New Member

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    Partially it could be that the tires are hitting a new patch of rubber and once that wears down you should see the penalty go away.

    In any event your tires should last a little longer running at a higher pressure.
     
  11. rrolff

    rrolff Prius Surgeon

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    You should never inflate to the max rating. When driving the car, the tires will heat up and exceed the max... 4PSI below the max should be safe...
     
  12. xs650

    xs650 Senior Member

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    The sidewall max number is inflation pressure for tires at ambient temperature. It doesn't matter if the tire pressure go higher from driving.
     
  13. Codyroo

    Codyroo Senior Member

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    I imagine that there is a TON of leeway on the top end pressure of the tire before you'd have issues with overinflating the tire. In other words, the stated maximum pressure may be 44 PSI (at "cold" conditions), but you likely wouldn't see issues with the tire unless you inflated to 60 PSI at cold temps. (just to avoid lawsuits).

    That being said, many the "extreme" hypermiler greatly exceeds the max sidewall PSI. I've heard DiamondLarry really really exceeds it (70 - 90 psi?). He hasn't blown up yet.

    For what its worth. I've been running max PSI on my cars for the last few years and I am not seeing wear on the inside or center of the tires. Further, I'm not seeing wear on the outside of the tire (a sign of underinflation).
     
  14. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    engineers are FULLY aware of that fact. tires are tested to be fully safe at 50% higher than max pressure.

    temps are a relatively minimal concern for engineers anyway. they mainly design tires to run at a certain pressure to withstand road conditions like potholes, etc. which is a much tougher test.


    now, would inflating tires to max pressure make them more prone to blowouts if encountering a pot hole big enough; probably. but also keep in mind, that pressures too low cause more problems and lower efficiency.

    best bet; pump em up and watch where you are going
     
  15. Insight-I Owner

    Insight-I Owner 2006 Insight-I MT + 2011 Prius

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    Actually no, this is not true. At higher pressures the tires flex less. As a result, they heat up less, so there is less pressure buildup. On another car, I inflated the tires to sidewall max 51 psi cold, drove at highway speed for 1.5 hours on a warm summer day, and then remeasured the tire pressures using the same gauge. They were 51psi - no change.

    If tires are UNDER inflated yes they will flex a lot and heat up, and this can weaken the tires and lead to failures (aka blowouts).
     
  16. alfon

    alfon Senior Member

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    For the last 18 years I have inflated my tires to max sidewall pressure on all my cars;

    1994 Honda Civic VX (this was one of the first true 50 mpg Honda's with seating for 4)

    2003 VW Jetta GL / TDI / 5-speed manual / station wagon

    2010 Prius

    My tires have always worn properly and I believed with running at max pressure I obtained max tire tread life on all my vehicles.

    al
     
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  17. fuzzy1

    fuzzy1 Senior Member

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    Do you also leave additional margin for elevation change? The difference between sea level and Vail Pass is almost 5 psi, in addition to any cold vs highway-hot difference.

    When getting some service on a very long vacation, a shop in a town at 6000 feet insisted that my tire pressures were 'dangerously high' and required them to be lowered to 40 psi. Two mornings later, cold at sea level, the tires were at 33 -- placard minimum for the rears, underinflated for the fronts.
     
  18. xs650

    xs650 Senior Member

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    Fortunately high altitudes are usually cooler than low altitudes so the tire pressure gain with altitude is partially canceled by the cooler temperatures. Fuggedabout it unless you are planning on moving to and staying at high altitude.
     
  19. fuzzy1

    fuzzy1 Senior Member

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    I'm also intentionally fuggedingabout rrolff's insistence on leaving 4 psi margin for highway heating too, as I've always understood those numbers to be cold inflation figures.
     
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