Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why do SUVs see fewer MPG on the Highway?

  1. Mar 6, 2012 #1
    Seems like a dumb/basic question; but, I want to totally flush out the answer.....as there's more to it than most people think.

    So, when I drive my SUV 250 miles to Las Vegas...I get 20 mpg. Many cars will get 30-35. What extra work is being done, and/or what inefficiencies are being experienced in my SUV versus the passenger car (and I'm focusing on the highway part...not the start/stop parts)?

    Here's what I've boiled it down to:

    * Acceleration- even if we ignore the energy required to accelerate from a stop, and only look at "highway" mpg.....a vehicle is still always accelerating between different speeds, even when you think you are "cruising" at a constant speed. The more you Δ speeds in a heavier vehicle, the more you're exposing the advantage of the lighter vehicle.
    * Climbing (accelerating against gravity)- the larger heavier vehicle must do more work when climbing a hill, assuming the same speed as the lighter/smaller vehicle. The more hills involved, the more the larger/heavier vehicle is being shown to be at a disadvantage. This is just another form of accelerating.
    * Aerodynamics- The larger vehicle (usually) must displace more atmosphere per hour (assuming same speed as the smaller vehicle). IF the coefficient of drag for the heavy SUV and the lighter passenger car are the same....then this will not matter....but that's a big "if".

    So, the weight of the vehicle itself doesn't automatically cause the problem (in terms of reduced highway MPG).....if you could maintain a constant speed and experience no hills, then you'd be more or less on par with a lighter vehicle. Right? In fact, two identical vehicles traveling at a perfectly constant speed (in theory), on perfectly flat ground.....but one with 1,000 Kg of bricks inside, while the other is empty, both should get roughly the same MPG over a 1000 mile trip. Yes?
     
  2. jcsd
  3. Mar 6, 2012 #2
    You've not taken into account that you have to do work against friction to maintain a constant speed. Friction is proportional to the normal reaction force, which weight does have an impact on.
     
  4. Mar 6, 2012 #3

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Are you assuming the same engine? If not, then that also makes a difference. A larger engine might use more fuel for the same work.
     
  5. Mar 6, 2012 #4
    Wouldn't that be (for most modern vehicles) very very very very slight....as a portion of the total extra work being done? I thought of this, which was why I used the word "roughly". I don't think increased friction due to increased weight will matter much. If that's not right....correct me.
     
  6. Mar 6, 2012 #5
    well, I wanted to leave the concept of the engine out for now...so I guess let's assume identical engines.
     
  7. Mar 6, 2012 #6

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Mass certainly counts, as you say, and less on the flat or at constant speed.
    The aerodynamics count a lot but it's mainly the shape and cross section, rather than the volume.
    Also, you have those great knobbly tyres, in which the treads flex every time they 'claw' against the road, producing a lot of heat. It's this thermal problem which accounts for the reduced maximum recommended speed for off-road tyres (it's stamped on the side of all tyres aamof). Racing cars use slicks when the road's dry, to reduce this drag of tyre on road.

    The fact that the Engine is bigger (probably) doesn't help because a heavy right foot can suddenly cause the engine to gobble up a lot of fuel even if you aren't trying to accelerate. Cruise control is quite brainless and will try to maintain your speed up a gentle slope and then hold you back down the other side, when an 'intelligent' driver will make use of downhill stretches. In a heavy vehicle, this will be more significant. Driving without cruise control in the US could annoy other users, though!
     
  8. Mar 6, 2012 #7

    Pengwuino

    User Avatar
    Gold Member

    A quick google search will show you that an SUV can be almost twice as heavy as a 4-door sedan. That is far from "very very very very" slight amounts of extra friction.
     
  9. Mar 6, 2012 #8
    weight = friction? :confused: I thought weight can influence friction, based on a host of other factors. In the example I gave (two identical vehicles, one empty, the other with 1,000 kg of bricks), I would estimate the increase in friction resulting from the increase in weight to be extremely slight, and thus not the primary (or secondary or even tertiary) cause of the reduced MPG. ???
     
  10. Mar 6, 2012 #9

    Office_Shredder

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

  11. Mar 6, 2012 #10
    So, if we assume that the SUV will be heavier and displace more air, it seems to me that the killer CONTROLABE variable, on long trips, is changing speeds. This is where the extra mass kills ya, as it (mass) must be accelerated. The killer UNcontrollable issues which are present for the SUV vs the passenger car will be atmospheric drag (inherent in the larger displacement....assuming this is the case) and gravity (when climbing a hill thus accelerating against said force, again assuming the SUV will be more massive, thus more work to be done).

    Just flushing it out.....and getting to the basics of why trains are the most efficient method to transport freight over distances. Once it's moving.....the weight becomes of very little importance (unless climbing mountains...which freight trains generally avoid....for this reason).

    Thanks!
     
    Last edited: Mar 6, 2012
  12. Mar 6, 2012 #11

    Pengwuino

    User Avatar
    Gold Member

    The friction between the tires and the road is directly proportional to the weight of the vehicle. This is certainly not the only cause of friction, but it is a significant, first order, factor. Your estimate is simply not correct. This actually gives me an idea for a lab for intro physics students...

    If you ever looked at the shape of a train's wheels, the composition of the wheels, and the shape of the track, you might be able to see why. The wheels are smooth and made of metal as are the train tracks. The surface area between the track and train wheel is very small as well. This combines to create a much smaller coefficient of friction. With a car, you have a very large contact area between the wheels and road, neither of which are smooth.
     
    Last edited: Mar 6, 2012
  13. Mar 6, 2012 #12

    turbo

    User Avatar
    Gold Member

    My wife is driving an SUV. Granted, the Subaru Forester is not a hulking Chevy or Ford, but it has impressive cargo capabilities, full-time all-wheel drive with traction control and stability control, and it beats any truck I have ever owned in the snow and ice. It gets ~27mpg. At less than $20K with a Sport-Shift Automatic transmission (you have to negotiate to get such a price), it ought be on the radar of anybody looking for a capable vehicle (carrying passengers and cargo) that will have to handle well in foul weather.
     
  14. Mar 6, 2012 #13
    I hear ya. I understand that a train has virtually eliminated the wheel to track friction element...at least per unit of weight hauled. And, for road vehicles, I understand that the friction of tires to road is proportional to weight, but I just can't see it being in the top 3 in terms of variables causing an SUVs highway mileage to suffer vs a "passenger car". Accelerating the extra mass on flat; accelerating mass against gravity and wind drag due to the larger vehicle surely are #1 #2 and #3..??...and probably (just guessing...but someone would have to really do some convincing for me to think otherwise) account for 80+% of the observed reduced mileage per gallon of fuel with a SUV vs passenger car.

    I have an "over the road" trucker friend (interesting business by the way). He says he gets, on flat ground, 11-12 mpg full (50,000 lb load). He gets 12-14 empty. Now, that's only a ~15-25% difference....and of that, how much is NOT due to accelerating the heavy load (thus is caused by the increased road-tire friction)? I don't know...but it seems like most of it is due to the acceleration of mass. Tires matter....but if you're keeping them at a good pressure (I go 40-42 psi on trips), just can't imagine that's where the weight, by itself, gets ya. Maybe I'm off on that..... I'm open to evidence. :smile:
     
  15. Mar 6, 2012 #14

    Pengwuino

    User Avatar
    Gold Member

    What do you mean by extra mass on flat?

    Can you be a little more specific and clarify what you said? Are you saying that without a load, he gets 12-14MPG while without a load, he gets 11/12MPG?

    I'm in a hurry at the moment, but it would be necessary to know exactly how much a truck weighs by itself. A situation with a semi would be more complicated because the aerodynamics are, I assume, considerably worse than a passenger vehicle which would cause wind resistance to be a greater factor.
     
  16. Mar 6, 2012 #15

    Drakkith

    User Avatar

    Staff: Mentor

    Do differences in gear rations matter much? For example, my motorcycle hits about 4k rpm+ at 60 mph on the highway compared to barely 3k that my truck gets. If another bike was geared to only need 3k rpm at the same speed how much of a difference might that make?
     
  17. Mar 6, 2012 #16
    I don't think so...but that is a good question, to which I'd love to know the answer. I see a bunch of paradoxes there:

    1) my 1000cc v-twin bike gets 40 mpg on the highway
    2) my 650cc single-cylinder bike gets 52 mpg on the highway....and is LESS aerodynamic while spinning at far higher RPMs.

    The v-twin displaces more in total, but less per cylinder than the 650. The 650 spins faster. The 650 is lighter (but that shouldn't matter much on the highway). The 1000 is more aerodynamic. Lots of contradictions there.
     
  18. Mar 6, 2012 #17
    What I mean by "on flat acceleration" is the acceleration of a mass on flat ground....the increase in speed (as differentiated from maintaining a constant speed while going up a hill...which is actually the same as accelerating against the constant force of gravity). An example of "on flat" acceleration would be a drag racer moving his 2,000# car from 0-110 mph is 10 seconds. An example of "against gravity" acceleration would be cruise control at 70 mph...while climbing a 10% grade.

    The semi-truck example is 12-14 MPG without load (if he dropped off, and didn't find a load for the return trip) on basically flat terrain. With the 50,000# load, on basically flat terrain, he gets 11-12 mpg.
     
    Last edited: Mar 6, 2012
  19. Mar 8, 2012 #18

    Pengwuino

    User Avatar
    Gold Member

    The engine is causing an acceleration in both situations. The engine (and thus, the tires) must create a force to propel the vehicle against the frictional force trying to slow it down. The only difference in going up a hill is that it has to create a greater force to push it up the hill.

    I can't imagine why your friend would get such poor performance without a load. The only thing I can think of is the aerodynamic resistance is a lot more important in a semi than in a car.
     
  20. Mar 8, 2012 #19

    cmb

    User Avatar

    I wish I could simply think up a concept, and it'd make my car go forward!!!

    Sorry, but your car is heavy, so it needs a bigger engine. At 40 to 50 mph you'd be generally right that mass and even aerodynamics over the top/sides of the car don't actually play such a huge part as people make out. But especially if you have the drag of 4 WD (even if it is in 2WD select), and the internal friction of the engine, then you will see that fuel consumption spiral.

    The tyres also play a big part at that speed, as has been mentioned. You could reduce their effect by using skinny tyres to a high inflation pressure. It also helps to have the largest rolling radius.

    Back to the aerodynamics again, the cross-section of your tyres actually contributes too - having such a big clearance off the road surface, and usually extra paraphernalia underneath the floor pan, leads to bigger aerodynamic issues than over the body of the vehicle itself. This is why you will note that the most fuel efficient cars have lowered suspension and under-body panels.

    Oh, and incidentally, you misunderstood the term 'coefficient'. The Cd relates to the ratio of aerodynamic drag to that of a brick of identical size to the vehicle, so if you have two cars with the same Cd then their aerodynamic resistance is then proportional to their frontal area - inclusive of the extra road height.
     
  21. Mar 8, 2012 #20

    Drakkith

    User Avatar

    Staff: Mentor

    Well, it is only 1 cylinder. I doubt it's spinning 75% faster at a given speed.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Why do SUVs see fewer MPG on the Highway?
Loading...