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Any Data on Tire Inflation Benefits?

Discussion in 'Gen 3 Prius Fuel Economy' started by Dog Face, Mar 6, 2012.

  1. Dog Face

    Dog Face New Member

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    So, I've read a number of posts on tire inflation and it seems like the consensus here is that increasing the PSI above the manufacturer's recommendation will provide a significant improvement in fuel economy. But where is the data to support this? And if it does improve mpgs, by how much does it improve on average per psi increase above recommended levels? Data I have seen only measures benefits of inflating an under-inflated tire:

    According to the us gov, "You can improve your gas mileage by up to 3.3 percent by keeping your tires inflated to the proper pressure. Under-inflated tires can lower gas mileage by 0.3 percent for every 1 psi drop in pressure of all four tires. Properly inflated tires are safer and last longer." (Gas Mileage Tips - Keeping Your Vehicle in Shape)

    But, again, this is only referring to the benefit when you inflate tires that are "underinflated", i.e., inflated below the manufacturer's recommendation. I have been unable to find a study that looks at benefits of increasing psi above recommended levels. And why should we assume that there is a linear relationship between increased PSI and increased MPG? Perhaps after you reach the manufacturer's recommended level, the benefit goes away or diminishes substantially.

    With many Prius owners dramatically over-inflating their tires (some even well beyond the manufacturer's limit on the sidewall) I would think that there would be some quantitative data to support this. Is there a study out there that I am missing?

    I won't get into the downsides of over-inflation, but they are there and must be weighed against the advantages. My issue is that it seems difficult to quantify the advantages.
     
  2. jdcollins5

    jdcollins5 Senior Member

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    Search on here for tire pressure. You should find plenty of data to support higher gas mileage with increased pressure.
     
  3. Dog Face

    Dog Face New Member

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    I have, and I read through a few threads discussing tire pressure; however, none addressed the issue I raised. The reason I made this post is because it seemed like a lot of folks just assume that there is a linear relationship between PSI and MPG and that statistics regarding benefits from increasing PSI on under-inflated tires apply to over-inflated tires.
     
  4. ksstathead

    ksstathead Active Member

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    No one I've seen says the benefit is linear. Even below placard, the .3% would not really be linear even though they dumbed it down to one number.

    You might find some 'studies' over at cleanmpg.com.

    Up to max sidewall, I would argue the only principal downside of higher psi is increased NVH (noise, vibration, and harshness), and its side effects (e.g., rattles).

    Above max sidewall, many find the extra fuel savings too small to pursue. Some go for all the fuel savings they can get. I cannot recommend going over max sidewall, but burst pressures are well above those. I find the NVH at or near max sidewall is something I have adjusted to, but am not interested in going much over.

    Tire wear, handling, mileage are all increased with more psi up to max sidewall.

    Suggest you try a controlled run at placard with trip B (round trip), then repeat at max sidewall and report back. I can say maxsidewall is significantly better than placard, but I choose not to make controlled runs to quantify it like we'd all want to see, as my focus is on fuel consumption, not mpg.
     
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  5. F8L

    F8L Protecting Habitat & AG Lands

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    You need to do the same search on clean mpg.com and ecomodder.com. That is where many hypermilers go for information.

    Higher pressure is better but you reach a point of diminishing returns somewhere between 45 and 50psi depending on the tire. There is a good thread on ecomodder with coast down tests at different pressures.

    You can also perform the tests yourself by finding a nice long flat section of road and doing coast down tests. I guarantee that if you perform the tests correctly you will roll further with overinflated tires. :)
     
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  6. Dog Face

    Dog Face New Member

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    @ ksstathead & F8L: Thanks--I'll check out those sites (mpg.com, ecomodder.com., & cleanmpg.com.) and report back. Unfortunately, I don't really have the time to run a controlled experiment on my own. (Between working full time, a part-time MBA program, and a wife and dog, I'm already working on my first ulcer).

    Regarding ksstathead's point, I suspect that you may reach diminishing returns on increased fuel economy shortly after exceeding the manufacturer's recommended PSI, but I'll see what I can find. I personally don't want to go crazy over-inflating my tires and dealing with discomfort and possible safety issues (some sources site significant increases in minimum stopping distances) if I'm getting an insignificant MPG benefit.
     
  7. F8L

    F8L Protecting Habitat & AG Lands

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    That is part of the reason so many of us just stick to sidewall max pressure. It gets you the bulk of the mpg gain, reduces harshness of ride and in some cases helps promote more even treadwear. Keep in mind that some tires like the Goodyear Fuel Max are rated at 50psi max sidewall as are most 17" 40-45 series tires.
     
  8. Former Member 68813

    Former Member 68813 Senior Member

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    There was a mythbusters episode on this. IIRC, they got like 20% boost going from underinflated to overinflated. I would not call it scientific though.
     
  9. Former Member 68813

    Former Member 68813 Senior Member

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    I would like to see evidence of that.
     
  10. ksstathead

    ksstathead Active Member

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    My cars at max sidewall come to at least as fast a stop as they used to at placard, though I rarely must 'test' it.
     
  11. Dog Face

    Dog Face New Member

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    Most of the analysis I've found on this issue seems to be comparing under-inflated to properly inflated tires, and the consensus is that properly inflating tires reduces stopping distance. But, as for safety-related downsides of over-inflating, I found the following doing a quick search. That said, I haven't found any really convincing analysis using scientific methodologies.

    JD Power & Assoc: “Tires that are over-inflated result in reduced levels of handling performance because less rubber is in contact with the road.â€
    Automotive: Why Tire Care Is Important | J.D. Power

    “If tire pressure is too high, then less of the tire touches the ground. As a consequence, your car will bounce around on the road. And when your tires are bouncing instead of firmly planted on the road, traction suffers and so do your stopping distances.†–Car Talk

    "Over-inflating your tires - whether to affect their feel and performance or stay ahead of a slow leak - is not a smart option. A tire that has too much air in it will have less contact with the road. While that means less friction and rolling resistance - a good thing - it also means reduced vehicle handling and longer stopping distances - not-so-good things.
    Also, over-inflated tires wear quicker and more unevenly than properly inflated tires. So, even though you may save a little gas money by over-inflating, you’ll pay more in the long run to replace your tires more frequently." Don
     
  12. Dog Face

    Dog Face New Member

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    Another fairly compelling article on the downsides of over-inflating:

    Over-inflated tires are a bad idea | Wheels.ca

    Excerpt:
    "On the lateral-grip (how well the car holds in the corner) aspect of overinflation, Bellissimo concludes,
    "If pressures are increased without the corresponding increase in load (adding weight), lateral grip will generally decrease. For vehicles in the market place, the placard inflation generally provides optimum handling."
    The Michelin engineering team ran a computer simulation on a family car with pressures increased to 44 psi from the proper 35 psi.
    They report, "Increasing inflation pressure can negatively affect performances such as handling, wear life, worn appearance and braking traction."
     
  13. mad-dog-one

    mad-dog-one Prius Enthusiast

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    One Dog to Another: I would also like to see numbers that evaluate the relationship between tire pressure and MPG, although there are so many mitigating (confounding) factors that relate to mileage and driving practices that it will be challenging to isolate the tire inflation effect. I would not expect the relationship between tire inflation pressure and MPG to be linear and, before your post, never encountered that presumption. I am well aware of tire damage done by over inflation, but have never encounter this in our Prii. I am unaware of any reliable data that link tire over inflation with crashes and found it ironic that, in your request for supporting evidence, you cite a government statistic. I love my country, but am suspicious of the political leverage that can bias government numbers. EG; the Chevy Volt averages 90+ MPG, if you don't have to pay for the electricity when you plug it in. 'I hope my neighbor doesn't see that heavy duty extension cord running from his garage.' :eek:
     
  14. Dog Face

    Dog Face New Member

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    Haha. Nice. So I think we agree that it would be good to see some statistics on MPGs versus PSI at a wide-range of different PSIs going above the generally recommended levels. Just one guy from that mythbusters article for example driving 400 miles in a honda insight at 70mph and saying there was no benefit is not good research because there are so many variables he wasn't controlling for. But you could do a study with the right data. Just give me the data and I'll do a nice big multivariate regression. The hardest part is just getting good data. And it seems the US government won't go out and generate this data because, unsurprisingly, they want to recommend that consumers use the recommended tire pressure and not to over-inflate.

    On a similar note, the NHTSA, for example, did a very nice study on tire pressure and stopping-distance, but only looked at a range between 17 psi and 35 psi, so they don't address over-inflating.
     
  15. jdcollins5

    jdcollins5 Senior Member

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    I raised the pressure on my OEM Yokohama Avid S33D's 2 psi at a time above placard until I got up to 44 psi max sidewall. I felt no decrease in handling performance but a noticeable increase in ride harshness. I also noticed a 1-2 mpg increase from placard.

    I ended up lowering mine back down to 40-38 because I did not like the harsh ride. I rotated tires every 5k miles. When I replaced these tires there was even tire wear from side-to-side and front-to-back. No noticeable decrease in mpg.

    I now have Michelin MXV4's at 40-38. After 10k miles tire wear is still even.

    So much for tire wear or handling issues due to overinflation.

    Of course this is not a scientific study by any means, just one person's opinion.
     
  16. Dog Face

    Dog Face New Member

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    Update: To put it bluntly, after doing more research, I found that a large percentage of folks on mileage efficiency forums over-inflate their tires (i.e. beyond sidewall maximums), but there is little beyond anecdotal evidence that doing so provides significant benefits.

    I did find a pretty cool article that charted glide/coast range at different PSI levels for 2 different vehicles. The findings suggest that you reach a point of diminishing returns (at 40 psi for 1 vehicle tested and 45 psi for another):

    Snapshot: effect of tire pressure on rolling resistance - MetroMPG.com

    Of course, as the author acknowledges, this doesn't constitute a conclusive study. I followed the research advice I was given and checked out cleanmpg.com, ecomodder, and other sources. I basically found a ton of anecdotal evidence and opinions, but nothing really in the way of a convincing evidence that you get a significant benefit by over-inflating tires beyond recommended levels. There also seems to be conflicting anecdotal evidence on performance/handling implications of over-inflated tires (some of which I site in earlier posts). In one survey on Cleanmpgs.com, they found that over 40% of respondents inflate their tires over 44 PSI:
    PSI % of respondents
    Under 35 8.16%
    [​IMG][​IMG]
    35 to 39 14.29%
    [​IMG][​IMG][​IMG]
    40 to 44 35.37%
    [​IMG][​IMG]
    45 to 49 11.56%
    [​IMG][​IMG][​IMG]
    50 to 54 18.37%
    [​IMG][​IMG][​IMG]
    55 to 59 3.74%
    [​IMG][​IMG][​IMG]
    60 to 64 4.08%
    [​IMG][​IMG][​IMG]
    65 to 69 1.36%
    [​IMG][​IMG][​IMG]
    70 to 74 1.02%
    [​IMG][​IMG][​IMG]
    75 or higher 2.04%
    [​IMG][​IMG][​IMG]


    So over-inflation seems very widespread amongst folks that use forums like these, but unless I'm missing something, there isn't much in the way of real evidence that it provides a significant benefit. And the possibility that over-inflation increases risk factors (as I said, there seems to be disagreement on this issue) leads me to believe that it's just not worth it unless I see some conclusive evidence.
     
  17. alfon

    alfon Senior Member

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    I have 195x65x15 Michelin Energy Saver tires with
    57,000 miles on them. All 4 tires are inflated to
    44 PSI which is maximum sidewall pressure as listed
    on the side of the tire.

    They are wearing perfectly. Will get another 10,000 miles
    out of them, and they seem to roll very smoothly.

    Last tank with nearly all highway driving (55-75 mph) was 52.6 mpg
    calculated, that is with temps 45-55 degrees, and 10% Ethanol Reg
    gas. In the summer it would have been well over 55 MPG calculated.
     
  18. JimN

    JimN Let the games begin!

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    I don't think you'll find the data you are looking for because owners don't drive on dynos.

    From High School Physics class: Friction is independent of surface area.

    Under- & over-inflation aren't numbers. An underinflated tire rides on its edges. The center does not make contact. An overinflated tire rides on the center. The edges don't make contact. If a tire is circular (not deformed) under load then it is overinflated.

    At max sidewall pressure the Goodyear Integrities on my 2006 still showed some edge wear. On dry pavement when braking the VSC light would sometimes flash but never on wet pavement.

    Set your tp at placard. How fast can you turn a corner? No braking. Are the tires squealing? Increase tp to max. sidewall. How fast now?

    "Driving Under Pressure" can be found here: Driving Under Pressure (full article) - CleanMPG Forums
     
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  19. Former Member 68813

    Former Member 68813 Senior Member

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    Let me put it this way:
    Every car tire I inflated to the factory specs had bad tire shoulder wear and sloppy handling. Things improved everytime I went over factory specs. Near 40 PSI is a sweet spot for me with perfect handling and wear pattern. It's clear to me that the factory settings are optimized for comfort and not handling. I don't do it for MPG as difference is minimal. I bet LRR tire choice gives a bigger MPG impact.
     
  20. fuzzy1

    fuzzy1 Senior Member

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    Many Gen2 owners took common tire wear problems and abnormally short tread life as evidence that the Prius placard recommendations were actually too low. Higher pressures improved the wear patterns and produced longer tread life. I think the jury is still out on Gen3, but many of us are not using the placard recommendations anyway.

    My previous two cars displayed similar treadwear signs of underinflation when run at placard pressure. Both produced more even treadwear at higher pressure, and the one still in the household is also getting better treadlife.

    One of them also displayed premature hydroplaning at placard pressure, becoming normal when I boosted the pressure. Then CleanMPG.com linked to a NASA and another aviation industry study showing that the onset speed of hydroplaning rises with the square root of inflation pressure. More pressure, less hydroplaning. Living in a rainy climate, this is a strong factor for me.

    After the Ford Explorer / Firestone fiasco produced many rollover fatalities, a too-low tire pressure recommendation was found to be one of many contributing factors. (Defective Firestone tires were the original headline, but numerous other factors came out later.) That busted my faith in those car manufacturer recommendations.
     
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