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Rolling resistance, tire tolerance, gas mileage

  1. May 3, 2015 #1
    A question in regards to rolling resistance. Am I wrong in thinking that not everything you hit on the road with your tire effects your gas mileage? Don't tires have a tolerance in terms of how much energy they can absorb before it effects your traction, therefore effecting your gas mileage?

    Hypothetically, if you were driving under controlled conditions where you had every single condition about the car in a constant state.. the amount of friction on the road, the engine's RPMs, the rate of travel, drag resistance, anything and everything you can think of about the car and it's travel remains the same. That is, until it hits a tiny bump.. such as a small coin on the road or just a tiny patch of sand. If the object is small enough, there won't be enough force to effect the traction of the tire as a whole.. and if it doesn't effect the traction, it won't cause you to use up more fuel. Or is that incorrect?

    Now I know that road conditions are pretty much in a constant state of flux.. and that is why it could be said that everything you run over effects your gas mileage, but if you were to create a controlled system like above.. will there not be a small limit of what you can run over that won't effect your mileage? If we created a controlled system and saw that a penny lying on the road doesn't effect your gas mileage, can't it be said that there is no guarantee a penny on the road will effect your gas mileage in everyday situation? I mean, it still could effect your gas mileage in an everyday situation by having a butterfly effect on the conditions that are constantly changing already.. but I don't see it as a guarantee. That is of course if there is a tolerance the tire itself can handle before effecting it's traction and therefore effecting gas mileage.

    Or would you say that everything on the road (aside from the road itself) would cause you to use up more gas when you run it over? If you were driving down a road where the friction was rather stable and hit a small, flat coin, would it automatically make you use more gas to keep your forward momentum?
  2. jcsd
  3. May 3, 2015 #2
    All tires have unique rolling resistance on all surfaces. The flexing of the tread to match the road surface and the sidewalls to match rim, various tires handle it differently. The suspension does more work absorbing "bumps" so that the loss of momentum of the vehicle is minimized. The effect of "imperfections" in the road isn't really of great consequence, not like a consistently rough road would be, for example dirt. At some speed air resistance becomes your biggest mileage factor.
  4. May 3, 2015 #3
    My question is, if all the conditions remained stable.. even the air resistance, would there be an increase in fuel consumption?

    Think of it like a thought experiment. You are driving down a road where the road is perfectly uniform, the air resistance is the same, the rolling resistance, tire pressure, RPMs of the engine, everything that you can think of that goes into calculating how much fuel you are consuming is the same. You figure out your fuel consumption and it is X amount of fuel per second.. and that amount remains constant because all the conditions going into your fuel consumption are constant. Then you run over a flat penny on the road.. will the act of running over the penny make you consume more fuel as you run it over?

    The reason I ask is because someone who says they are a physics teacher is saying that no matter what, running over something (no matter how small) will cause you to consume more fuel.. and I don't think it will. There is a tolerance to the amount of force a tire can take on before it will effect the engine, therefore effecting the fuel consumption. If the tire hits something and it can't handle the force and loses traction, I can see it causing an increase in fuel consumption.. but if it is something small like a coin, I just don't see it.
  5. May 3, 2015 #4
    Even something as small as a coin adds some resistance, but I don't think it would be measurable.
  6. May 3, 2015 #5
    But will that resistance be guaranteed to increase fuel consumption, even if it is an immeasurable amount?

    What if you were driving on a road that was smooth as glass and all the other factors remained constant.. then you ran over a single grain of sand. Would that single grain of sand cause a minute increase in fuel consumption?
  7. May 3, 2015 #6
    A grain of sand I doubt it.
  8. May 3, 2015 #7
    That's what I say, but I was told that according to.. well physics.. and assuming a uniform surface on the road, anything you run over on the road will either decrease your forward momentum or increase your fuel consumption to maintain your forward momentum. I don't think that is true because just like with the single grain of sand, the tire can handle a small amount of force without effecting the whole car. Am I wrong in thinking that?
  9. May 4, 2015 #8
    Everything that affects a wheel affects the entire vehicle. A grain of sand forces the wheel upward slightly, which pushes up the strut, which pushes up the vehicle, which diverts a slight bit of forward acceleration upward. Yes that is how physics works, it may seem so miniscule as to have no effect but I assure you it does.
  10. May 4, 2015 #9


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    I occasionally drive along an expressway that has a section of smooth new concrete pavement, and a section of very coarse asphalt (no actual potholes or bumps, simply a very coarse texture). My car's dynamic MPG indicator seems to show a slight decrease after I enter the coarse section. I haven't tested this rigorously, though. The MPG indicator displays the cumulative average since I last reset it, so I'd have to reset it for each section of road instead of just when I fill the tank, as I normally do.
  11. May 4, 2015 #10
    I think I thought of an example... as in the accumulative effect of sporadic patches of sand could "slow you down"... but which grain? When you feel a slight roughness to driving whatever amount of sand is required to have a detectable impact. Then every grain has the equal probability to slow you down... so every grain counts.
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