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High Tire Pressure = false MPG?

Discussion in 'Gen II Prius Fuel Economy' started by Mormegil, Oct 1, 2007.

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  1. Mormegil

    Mormegil Member

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    I just got my Prius last Sunday, and after reading posts here on better fuel economy, I decided I would try upping the pressure on the tires a bit.

    I wonder if some of the fuel economy could now be off.

    So we know that odometer readings are based on tire rotations. But if the tires are over inflated, could that throw off the circumferance of the tire, leading to less tire rotations covering the same distance. So, even though you cover say, the 50 miles, the car's computer / odometer thinks you did less miles (since less tire rotations are required). So could that give a false low MPG rating?
  2. efusco

    efusco Troll Slayer Staff Member

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    No, the tire circumfrence is not affected...at least not enough to have any measurable impact. If you somehow put enough in to make it buldge out in the center then maybe it would be a slight difference.
  3. bwilson4web

    bwilson4web 03 and 10 Prius

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    <div class='quotetop'>QUOTE(Mormegil @ Oct 1 2007, 10:18 PM) [snapback]520127[/snapback]</div>

    Our radial tires are especially strong and the circumference does change with pressure. However, when changing tires, it is worth while to check your mileage with the new tires.

    Find a road with mile markers and take a 50 mile trip noting the trip meter, fraction of a mile as you pass the markers. Over a 50 mile interval, you'll be able to calculate the percentage difference, a 'correction factor.'

    In an ideal world, you calibrate the old tires and put the new ones on and calibrate the new ones.

    Bob Wilson
  4. patsparks

    patsparks An Aussie perspective

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    Hey, where is the problem if it did give a false reading? The reality would be even better than you think you are getting and what you think you are getting would be better so hey, it's all good.
    Tyre age and wear would have a bigger affect.
  5. alexstarfire

    alexstarfire New Member

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    I'm sorry, but I just don't see how air pressure can affect your tire circumference. Perhaps I'm just a noob, but someone should try to explain that. The volume of the tire would be different, that's for sure, but the surface area, and the circumference, aren't going to change. The best way to demonstrate this would be for you to take a tire, with no rim or anything, just the tire, and run it over a set distance. All you'd have to do is mark the tire, so you can count rotations are such, and then run it over like 100 feet, or more, without putting pressure on the tire and then do it again but put a lot of pressure on it so that it looks like a flat tire or something. I'd bet money that you'd get the same amount of rotations.

    If you do over inflate the tire too much you could cause the tire to expand which would give you false readings, but it'd be like 1%-2% at worst. I'd be more worried about the tire blowing out at that pressure than false mileage readings.
  6. apriusfan

    apriusfan New Member

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    <div class='quotetop'>QUOTE(Alexstarfire @ Oct 4 2007, 10:39 PM) [snapback]521602[/snapback]</div>

    If you over inflate the tires to the point of failure of the tire, erroneous mpg will be the least of your worries.

    If you are concerned about whether you are truly getting improved mpgs from adding more air pressure, try this test:

    Run a calculated test with the factory recommended (35F/33/R?) air pressure. By calculated, I mean fill up the tank until the auto shut-off engages. Note your odometer reading at the fill-up. Then drive your normal trips until it is time to refill. Refill until the auto shut-off engages. Note gallons put in the tank and the current reading on the odometer. Divide miles driven by gallons filled for a baseline mpg number. Then bump the air pressure to something like 40F/38R and repeat the test. I think you will notice an improvement in the mpg number with higher air pressure in your tires. How high to raise the pressure is a personal decision (up to the maximum the tires are rated for). You will be juggling increased ride harshness (from the higher air pressure) against improved mpgs.
  7. donee

    donee New Member

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    Hi Apriusfan,

    Your fuel volume measurement technique is flawed. Because the Prius has a variable sized gas tank. The way to get an accurate fuel volume would be to divide the odometer miles driven by the mfd mileage (mpg). As miles (and the effect on the measured miles due to tire variation) are in the numerator and denominator, the effect of the tire variation cancells out, and you get accurate gallons.
  8. bestmapman

    bestmapman 2010 Prius

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    <div class='quotetop'>QUOTE(Mormegil @ Oct 1 2007, 11:18 PM) [snapback]520127[/snapback]</div>

    For that tire there should be no change in distance covered for each rotation regardless of tire pressure. Here is why. The tire does not slide on the rim. The tire tread on the outside of the tire is a fixed length. The steal belts in the tire do not let it expand. The circumfrence of the tire does not change. As you put more air in the tire the footprint or "flat" area where the tire meets the ground decrases thus the car raises and apears to get bigger.

    The bottom line is the tread is what matters. If when you put more air in the tire the tread would stretch, then the tread would get longer. That would affect rotations per mile. The steel belts and the banding prevent the tire tread from stretching. If the tire tread does stretch it is so small as not to be measurable.

    When you change tires that will change rotaions per mile. Also as the tire wears that changes the circumfrence and thus rotations per mile.
  9. kram

    kram New Member

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    <div class='quotetop'>QUOTE(bestmapman @ Oct 5 2007, 09:08 AM) [snapback]521668[/snapback]</div>

    Are you sure about that? Perhaps the steel bands in the radial tires do prevent the tread from expanding but I'm not sure. Also, I think it's likely that a flatter tire on a vehicle makes fewer revolutions per a given distance. The reason I believe this is based on experiments I did with my bike. I was setting up an odometer for my bike and since there are a myriad of different wheel sizes/tire sizes available for bikes the instructions call for a means to determine what the effective circumferance of one's tire is so as to properly calibrate one's odometer to one's bike. I performed the calibration procedure under three different scenarios:
    1) Filled tire pressure to max allowed value. Marked tire and ground; walked bike for two tire revolutions and measured the distance traveled.
    2) Same as above except I used about 20 psi less (about 20%)
    3) Same as 1 except I was on the bike while traversing the 2 revolutions

    Results:
    Longest distance traveled was with 1). Both 2) and 3) gave measurably less distance.
    Conclusions:
    At least for bike tires, degree of inflation directly affects the distance traveled per revolution and therefore affects the odometer reading. Same is true for the weight of the "vehicle". The difference between 1) and 2)-3) was about 0.5%.


    Actions:
    I used the distance in 3) to calibrate my bike's odometer.
    As yet I have not tried this with my Prius.
  10. apriusfan

    apriusfan New Member

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    <div class='quotetop'>QUOTE(donee @ Oct 5 2007, 03:54 AM) [snapback]521637[/snapback]</div>

    I must be missing something. How does the Prius have a variable sized gas tank? If you are referring to the bladder inside the tank, that is part of the evaporative emission control system, not a variable sized gas tank. When the tank is empty, it will hold something like 13 gallons of gas (every time the tank is empty).
  11. bestmapman

    bestmapman 2010 Prius

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    <div class='quotetop'>QUOTE(kram @ Oct 5 2007, 02:17 PM) [snapback]521864[/snapback]</div>

    The way to find out is to test it.

    I think the way to look at this is the tread on the tire is like a tank tread. A military tank has a tread that is directly linked to the axle. The tread is in contact to the ground on a wide footprint. It doesn't matter what the shape of the rest of the tread is, it is directly connected to the drive train and can take any shape. For a certain revolution of the crankshaft the tread moves accordingly the proportional but same distance.

    The tire does not slip on the rim. I think a radial tire acts similiar to a tank tread. The bottom of the tire has a footprint. The size of the foot print can be changed by tire pressure, but it has a footprint or like a tank tread, a flat area in contact with the ground. Since this flat area in contact with the ground is connected to the rim and does not move the tire pressure should have no impact on revolutions per mile.

    By contrast, your bike tire changes shape and doesn't have the same type of footprint that a radial tire has. If you look at the radial tire, It is designed to have alot of area in contact with the ground. It does not act like a true wheel with only a small area in contact with the ground, but instead acts like a mini tank tread. With lower pressure more of the tread is in contact with the ground. With higher pressure less of the tread is in contact with the ground, but the revolutions per mile should not change.

    This is based on the tread not streching with different tire pressures. If the tire tread streches and becomes longer then it will not hold true.

    Like I said, the way to find out is test it. Might be a fun way to spend a Saturday afternnoon.
  12. kram

    kram New Member

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    <div class='quotetop'>QUOTE(bestmapman @ Oct 5 2007, 03:40 PM) [snapback]521890[/snapback]</div>

    [​IMG]


    The way I think of this can be seen in the crude picture. In image A is my attempt at showing a flat or under inflated tire. Image B is a "perfect" tire or one that is greatly inflated. The black dot in each tire is the center or axle of the tire/wheel. Note that axle A sits closer to the ground than does axle B. The distance from the ground to the axle in each case is the effective radius. Rotating this radius around in tire A gives the dotted circle. This is the effective circumference for the flat tire. The effective circumference of tire B is that of the tire itself since it doesn't deform. Circumference of tire A is much smaller than that of tire B.

    Even with radial tires, as you deflate the tire the axle sinks closer to the ground and the effective circumference decreases. It would be the same as you added weight to the vehicle causing the tire to look more and more like A as the weight is increased. Therefore as the wheels turn each revolution of tire A will move the vehicle less far than tire B and hence affect the odometer reading. Underinflated tires (or over weighted ones) will travel less than the odometer shows (assuming the car company calibrated the odometer to properly inflated tires).

    One question I have is the exact relationship. Does a change of say 1 psi when the tire is at its maximum inflation have more or less effect on the effective circumference than 1 psi when the tire is sufficiently deflated? My guess is it doesn't and the relationship is non linear. Once the pressure is sufficiently high, there will be essentially no deflection in the tire and the effective circumference will be the same as the apparent circumference. Adding more pressure will not make it deform less at least up to a limit.

    And what overall is the effect on mpg of tire inflation? There are other factors that come into play such as rolling resistance. I wonder how well the studies were done when mpg was determined for tires under different inflation pressures.

    his line of reasoning does not require that the tread be able to stretch.
  13. donee

    donee New Member

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    Hi All,

    I agree with Mapman. The radial tire is like a tank tread that is pushed around circuit every wheel revolution.

    So, to a first order approximation, the traveled distance per revolution is the length of the tread. And the length of the tread does not change with inflation.

    What does change is the angle the side walls are flexed each revolution. And the bigger that angle (lower the tire pressure) the hotter the rubber in the side walls is going to get. So mileage will suffer with low tire pressure.

    Now, we are also talking down in the sub percentage accuracies. So, there very well maybe a second order variation in the distance travelled per mile. When I drove TSD (Time Speed Distance) road rallies , I made a speedometer chart. I used the highway mile markers and calibrated the speedometer and odometer. So, this was like two linear equations. The goal was to get Speedometer as a function of speed. But, your measuring that with Odometer, which has a slight variation with speed as well. So out on the highway I went and measured odometer, miles covered and speedometer all at the same time, and repeated at various speeds. Then I analyzed the data back at home. At least as best as I can remember. This was in 1979. I then combined it all into a chart, where the navigator would look up the desired speed, and tell me how fast to drive the speedometer. We usually nailed the rally master time driving in SOP class (no crazy racing ahead and using instruments to then coast across on time). We just drove the route. I did put a little extra speed on after turns, to make up for the slowness of the turn. But this was just done by guestimate. The Rallies were like 2 1/2 hours of driving. So, one had to be very accurate. The poor little Opel Manta was quite warm after one of these rallies.

    The question is then, does the MFD read true improvement, or better than true improvement, or worse than true improvement ? That depends on those second order distance variations. The faster you go the hotter the steel in the tread belts, and the longer the belts - is one example of a second order effect.
  14. blamy

    blamy Member

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    I had 28 psi in my tires from the dealer and around town I averaged a solid 44mpg over several tanks of fuel. I changed the pressure to 38/36 and have got an average of 49mpg over several tanks. At the end of this tank I will be going to 40/38 to see what that will do. I am hoping to average 50 or above and I think it is possible. I also noted that the ride is slightly harder but not so much as to cause concern.
  15. bestmapman

    bestmapman 2010 Prius

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    <div class='quotetop'>QUOTE(kram @ Oct 5 2007, 05:43 PM) [snapback]521953[/snapback]</div>

    Actually your drawing shows my point about the tread length remaining the same. In your B drawing it is obvious that the tire, when fully inflated, is connected to the wheel rim and does not move. The rotaions per mile can be calculated.

    In your A drawing, you state that because the hub or rim is closer to the ground that the effective radius is smaller. If this were true, then the outside tread would move slower than the new effective radius portion of the tire. This would tear the tire apart. The outside tread is still conneted to the rim via the sidewall. This new effect radius would turn faster than the tread. This is not possible.

    Therefore with the tread still connected to the rim via the sidewall, the tire will not have any change in revolutions per mile as still as long as the tire tread is in contact with the ground and does not slip.
  16. Per

    Per New Member

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    Actually Kram is quite correct in his analysis. What changes with an underinflated tire is the amount of thread in contact with the road. It should be obvious that in drawing A, the axle (and engine) would have to spin much faster at the same speed. Higher RPM alone would reduce mileage, and the added friction in the underinflated tire would also cause energy loss, not to mention heating up the tire and causing tire failure.
  17. JimboJones

    JimboJones New Member

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    The issue is really about rolling resistance, when talking about mpg.

    Auto tires do not appreciably increase their circumference as long as pressures are below that which cause the tire to bulge (not a good situation). Therefore, as long as the tire is inflated within specs, tire circumference is practically constant.

    However, underinflation causes the tire to flex a lot more than a fully inflated tire - this is when friction losses increase due to the underinflated tire flexing more, which also heats up the tire more. The tire is now burning off more energy which the engine is supplying. The fact that the tire has a slightly greater contact patch with the ground is a consequence of underinflation, and not the cause of lower mpg. Think about this - a larger tire diameter (such as that for a semi) does NOT necessarily mean higher rolling resistance than a smaller diameter tire, despite the larger diameter tire having more rubber in contact with the ground. It's all about rubber flex, which is greatly affected by tire pressure.
  18. Mormegil

    Mormegil Member

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    I guess that's a no then. Thanks for the replies. Got an interesting discussion out of it.

    So it looks like an underinflation wouldn't affect odometer readings. A super over-inflated tire, where the tread area is stretched, could lead to bad odometer readings... but that would the least of concerns.
  19. indigoblades

    indigoblades New Member

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    couple of points - one Kram is correct. I am Mech. Engr. and studied a lot of vehicle dynamics.

    Second effects of tire pressure do change the effective tire radius, so does more car weight

    Third the only tire error with higher pressure would be ur are getting more miles per gallon the mfd tells u because ur effect radius is bigger.

    . ...Effects of tire pressure are published and known by professional race mechanics (tuners).... i have Carol Smiths racing series and in the Book "Engineer to Win" or "Tune to Win" he goes in lots of detail about tires and they are not perfect circles once weight is put on them, they spin, accelerate, brake and corner. To convince Yourself look at a typical front well drive car with most of the weight on the front ... You will notice the the front wheels have the tire bulged out slightly further when both pressures are equal ... how much the bottom of the tire is deformed per Kram diagram determines the effective radius of the tire and it is less than half its diameter measured horizontally across the tire. Since the radius is less the car is gear a slight amount more and and travels less far ... if u add pressure to the tire the deformation is reduce which increases the effective radius. This larger radius makes u travel further (distance = 2*Pi*R.effective). Also ur acceleration should be slightly less if drive train gearing was constant. so if anything higher pressures actually give u higher mileage than u either compute or read on the mfd. i suspect its slight as get close to 40 psi. there are some centripetal effects too which reduce the deformation some, but thats not a function of tire pressure.
  20. JimboPalmer

    JimboPalmer Nadir of Wrongness

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    For the purpose of discussion, lets assume a tire with higher air pressure has a larger circumference. (No one thinks it could be smaller, right?)

    Since the assumed larger circumference for a given real distance, the tire will rotate less often, and the odometer will show a smaller distance. Same gas /smaller distance = lower MPG. So if it IS misreading MPG, it is artificially low. Not High.

    Me, I think the circumference is much the same. I try to inflate my tire until the wear is even, even wear implies maximum tread on the ground and best handling. Better MPG is just a side benefit.