Lifetime gas mileage calculation

[Yoda_One]

[Subisnob]

given the mpg's ive seen at the pump over the past 9 months, i wonder sometimes ...

this whole fiasco recently with vw and it's diesel cars ... anyone else think that other manufacturers partake in similar activities as what vw did with their software ?

 

VW fudged the tailpipe emisions. This is different than mpg fudging.

 

Subaru testsfor hwy mpgs are at 50mph and is where I noticed best mpg. Mpgs drop as sped goes up from there. To me, that is sorta fudging I mean, what highway is just 50mph?

[andrew_l_s]

VW fudged the tailpipe emisions. This is different than mpg fudging.

 

Subaru testsfor hwy mpgs are at 50mph and is where I noticed best mpg. Mpgs drop as sped goes up from there. To me, that is sorta fudging I mean, what highway is just 50mph?

 

Every highway is for left lane slow pokes:spin:

 

50-55 is the optimal speed for MPGs before drag really kicks in. EPA uses 55.

[zero7404]

50-55 is the optimal speed for MPGs before drag really kicks in. EPA uses 55.

 

 

your statement about drag is not accurate. you make it sound like after 55 mpg the aerodynamic drag force experienced by a car will increase dramatically, which it doesn't.

we are talking cars, not planes.

 

where did you get 55 mph anyway ?

[ammcinnis]

... you make it sound like after 55 mpg the aerodynamic drag force experienced by a car will increase dramatically, which it doesn't.

we are talking cars, not planes.

 

No. The air doesn't care. Aerodynamic drag goes up as the square of the airspeed for any solid body, whether it's an automobile, an aircraft fuselage, a Radio Flyer wagon, or a brick.

 

For example, aerodynamic drag is 62% higher at 70 mph than at 55 mph.

 

Drag normalized to 55 mph:

55 mph = 1.00

60 mph = 1.19

65 mph = 1.40

70 mph = 1.62

75 mph = 1.86

80 mph = 2.12

[Yoda_One]

Thank you @ammcinnis I was going to research the drag coefficient.

[ammcinnis]

Thank you @ammcinnis I was going to research the drag coefficient.

 

The drag coefficient of the 2015 Legacy is 0.29 (a constant) per Subaru ... the same as my 1969 Porsche 911S. Of course, the frontal area of the Legacy is larger, so its drag at any airspeed is somewhat higher.

 

drag ~ (drag coefficient) x (frontal area) x (air density) x (airspeed squared)

[zero7404]

what is the frontal area of the car ?

[Yoda_One]

You will need to research it yourself my friend. To have an intellectual debate you need to have fact on your side.

[zero7404]

ok ... exact area is not critical to show how the calc's work out, so i will choose 24 sq. ft. as a first guess. I can't find the actual frontal aerodynamic area published anywhere.

 

Cd = .29

Density of Air = 1.25 kg/m^3

Area = 2.23 m^2

Velocity (55 mph) = 24.6 m/s

Velocity (70 mph) = 31.3 m/s

 

at 55, the aerodynamic load is 245 N (55 lb).

 

at 70, the force is 396 N (89 lb.)

 

at 85, the force is 583 N (132 lb.)

 

at 100, force is 807 N (182 lb.)

 

load increases with speed but the difference between adjacent loads is getting smaller.

[zero7404]

regarding drag ... in general its not the biggest contributor to mpg degradation.

 

if rpms at 55 are higher than at 70, you are burning more fuel at 55 by some small amount. vice versa if rpms are lower at 55.

 

so i wouldnt rely on a single parameter as the focus of this subject.

 

i think fuel consumption is not largely impacted by aerodynamic force because the aerodynamic forces are quite small.

[ammcinnis]

at 55, the aerodynamic load is 245 N (55 lb).

 

at 70, the force is 396 N (89 lb.)

 

at 85, the force is 583 N (132 lb.)

 

at 100, force is 807 N (182 lb.)

 

load increases with speed but the difference between adjacent loads is getting smaller.

 

Your conclusion makes no sense. For example,

 

85 mph to 100 mph = 18% increase in speed

 

132 lbs to 182 lbs = 38% increase in aerodynamic drag (1.18^2)

 

---------------------------------------------------------------------------

 

FWIW (using your numbers):

 

89 lb drag @ 70 mph = 16.6 hp (due solely to aerodynamic drag)

 

132 lb drag @ 85 mph = 29.9 hp

 

182 lb drag @ 100 mph = 48.5 hp

 

regarding drag ... in general its not the biggest contributor to mpg degradation.

 

... and you would be wrong.

 

See: http://ecomodder.com/forum/tool-aero-rolling-resistance.php?Weight=3500&WeightUnits=lbs&CRR=.008&Cd=.29&FrontalArea=25&FrontalAreaUnits=ft^2&FuelWh=33557&IceEfficiency=.22&DrivetrainEfficiency=.80&ParasiticOverhead=0&rho=1.225&FromToStep=5-200-5 for an example calculation that is a very close approximation to the 2015 Legacy. At 70 mph, aerodynamic drag is responsible for ~75% of the total engine load.

 

Edit: On ecomodder.com calculation, increased frontal area to 25 sq ft and reduced drive train efficiency to 80% to better represent the 2015 Legacy. Results were largely unaffected.

 

if rpms at 55 are higher than at 70, you are burning more fuel at 55 by some small amount. vice versa if rpms are lower at 55.

Power ... and fuel consumption ... is primarily a function of (rpm) x (absolute manifold pressure), not rpm alone. Yes, engine friction and pumping losses are higher at high rpms, and engine efficiency is generally best at low rpm/high manifold pressure. For a given power output, lower rpm is generally more efficient.

 

so i wouldnt rely on a single parameter as the focus of this subject.

Did anyone actually suggest that? I was simply responding to your assertion that, "... you make it sound like after 55 mpg the aerodynamic drag force experienced by a car will increase dramatically, which it doesn't."

 

i think fuel consumption is not largely impacted by aerodynamic force ...

See the "% Aero" and "MPG" columns at the link I provided above.

 

See also: http://www.fueleconomy.gov/feg/atv.shtml

 

... aerodynamic forces are quite small.

Tell that to the passengers and crew of an Boeing 787 airliner, with a max takeoff weight that exceeds of half a million pounds and a wing loading of ~140 pounds per square foot.

[SubieN8]

[ammcinnis]

 

Management Summary:

 

1) Below ~40 mph, drive train friction and tire rolling resistance dominate.

 

2) Above 40 mph, aerodynamic drag is increasingly (i.e exponentially) dominant.

[SubieN8]

^^^ I agree and I think it makes sense to everyone. Drive sensibly and enjoy your Subaru's efficiency. And always remember, it's not a VW, MB or BMW.

[Yoda_One]

Just average over 33 mpg on a 107 mile run round trip to try a Restaurant. The wife was using her laptop to complete patient information and commented how smooth the ride and no motion sickness which she expected.

[Yoda_One]

It's PacMan time. :munch: : munch:

Or should change Man to: person, individual,non female/male, robot. This PC S___t is a PITA.

[andrew_l_s]

where did you get 55 mph anyway ?

 

I think the argument has been handled but I got 55 as the EPA hwy mileage test speed from the EPA's website, and the 50-55 optimum speed part is something I learned forever ago, if you google "optimum speed for highway fuel economy" you'll get plenty of sources.

 

 

Management Summary:

 

1) Below ~40 mph, friction and tire rolling resistance dominate.

 

2) Above 40 mph, aerodynamic drag is increasingly (i.e exponentially) dominant.

 

Thanks for all the research and calculations, as someone going into engineering I can appreciate the thoroughness:) also helps win arguments too, not that anyone was disputing it:rolleyes:

[zero7404]

it seems like this is turning into a dick slapping match ... believe what you want based on an ecomodder forum discussion. so you would be right if you feel their discussion is accurate and closer to the gospel that you align with.

 

i found discrepancies in their assumptions, so my opinion still remains the same. i also observe what my car has been doing with fuel consumption since day 1 and i dont feel that aerodynamics has any real bearing on mpg as it pertains to motor vehicles in general since the speeds and loads are insignificant. furthermore if you believe that drivetrain efficiency is 95% good for you.

 

i dont have that tendency to keep coming back and arguing in order to come out right in the end, like some ppl in here .... so i'm going to ignore this discussion and move on. good luck.

[SubieN8]

^^^

[ammcinnis]

... my opinion still remains the same ... i dont feel that aerodynamics has any real bearing on mpg as it pertains to motor vehicles in general ...

 

1) Not all opinions are equally valid.

 

2) The laws of physics are rigidly enforced.

[zero7404]

noted ...

 

today i tested my mpg on a daily basis and found i made about 36-37 mpg with continuous speed of 60-70, all highway, for 61 miles.

perhaps you could describe what you think about that ?

[andrew_l_s]

it seems like this is turning into a dick slapping match ... believe what you want based on an ecomodder forum discussion. so you would be right if you feel their discussion is accurate and closer to the gospel that you align with.

 

i found discrepancies in their assumptions, so my opinion still remains the same. i also observe what my car has been doing with fuel consumption since day 1 and i dont feel that aerodynamics has any real bearing on mpg as it pertains to motor vehicles in general since the speeds and loads are insignificant. furthermore if you believe that drivetrain efficiency is 95% good for you.

 

i dont have that tendency to keep coming back and arguing in order to come out right in the end, like some ppl in here .... so i'm going to ignore this discussion and move on. good luck.

 

This isn't politics...there's no taking sides or opinions. It's tested and proven science. May I ask what makes your observations of your car's fuel consumption more credible/accurate than physicists' published work on fluid dynamics, and the auto industry's wind tunnel testing?

 

I'm not trying to be rude or anything, but you're trusting what your eco-monitor says (which has been noted by many to be 1-2 mpg off) over scientific research done by 1,000s of credible physicists, engineers, etc...seems a bit off...

 

noted ...

 

today i tested my mpg on a daily basis and found i made about 36-37 mpg with continuous speed of 60-70, all highway, for 61 miles.

perhaps you could describe what you think about that ?

 

So have I, on a trip to Indiana at an almost constant 65 I averaged 37.5.

In order to simulate real-world conditions, the EPA rating is not done at a constant 55, hence why you can get the rated 36 hwy at speeds above that. I've reached 40 doing a constant 55.

 

Attached is a graph showing the amount of horsepower (and therefore, fuel) needed to overcome different forces of drag across a range of speeds. You can see that drivetrain losses slightly increase with speed, rolling resistance slightly more so, and aerodynamic drag is definitely the biggest piece of the pie. According to the article, this data comes from research done by Volvo. https://www.quora.com/How-much-does-aerodynamics-affect-fuel-efficiency

 

If you still don't believe higher speeds result in more air resistance, try this kiddie science experiment for yourself. Get a good sized sheet of corrugated cardboard (over 1'x1') hold it with your arm fully extended to the side. Slowly move it up and down like a bird flapping it's wing. Easy right? Now move it as fast as possible. Notice a difference? A resistance to motion? Hear the rushing of air past it?

Air is a fluid. It will move out of the way for you but only reacts so fast. It reacts to slow changes just fine, however fast movements happen quicker than the air can make room, causing drag as an object slices through.

 

If you can cite some scientific proof that supports your claims I'm sure I'm not the only one that would like to see it:)

[zero7404]

i dont believe that higher speed result in much higher loads, as you said the drag is much more above 55 mph, which is just not the case.

 

there is obviously increased drag at higher speeds, its just not overblown as someone would think when you say as such.

 

not disputing science and fluid mechanics, i know the field well. i majored in engineering and took fluid dynamics courses at advanced levels.

 

i pointed out the drag calcs before and its nothing dramatic and i dont see it being a major contributor to fuel consumption in the case of land based vehicles. i work in aerospace, where every ounce counts and coefficients of drag are painstakingly worked thru design and test to produce the most efficient systems possible.

 

 

sea borne and air borne vehicles are different and drag is more in the forefront of fuel consumption. however, on the ground, drag takes a back seat. there are cars out there with higher Cd's and are able to put out respectable mpgs, even at higher speeds.

 

there are lots of other factors and my speculation related to this model car is that it has to do with poor software/hardware integration and/or poor fuel management/drivetrain efficiency.

 

and this car does not have a 95% efficient drivetrain, as the link to ecomodder suggests.

 

this is my perspective on the topic, and as pointed out earlier, its just an opinion.

in the end we made our points here on this, i dont want to reverse engineer the car, although it would be interesting to learn about it.

[ammcinnis]

today ... i made about 36-37 mpg with continuous speed of 60-70, all highway, for 61 miles.

perhaps you could describe what you think about that ?

 

That is generally consistent with what I achieve on the highway in my 2015 Legacy 2.5. This past weekend I drove a 680 mile round trip. On the way down (340 miles) I recorded 30.6 mpg at 60 to 70 mph in light to heavy rain showers. On the way back (340 miles, same route) I achieved 37.8 mpg at 70 mph on clear and dry roads. This summer on a 3,300 mile road trip my wife and I averaged 35.6 mpg overall at highway speeds of 55 to 80 mph and elevations ranging from 1,000 to 11,500 feet. YMMV.

 

http://badges.fuelly.com/images/sig-us/311503.png

 

i dont believe that higher speed result in much higher loads, as you said the drag is much more above 55 mph, which is just not the case.

...

i know the field well. i majored in engineering and took fluid dynamics courses at advanced levels.

 

i pointed out the drag calcs before and its nothing dramatic and i dont see it being a major contributor to fuel consumption in the case of land based vehicles.

While you claim to have studied the subject ... which I have no reason to dispute ... you seem not to understand it very well. What is so difficult to understand about drag being a function of velocity squared?

 

I seldom do this unless provoked, but since we're whipping out credentials: I was educated as an aeronautical/aerospace engineer; I hold a commercial pilot's license with airplane, glider, and instrument ratings; I've built an airplane with my own hands; I've been wrenching on my own vehicles (cars, motorcycles, and airplanes) since the late 1950s. I owned and managed my own technical consulting business for 40+ years, and my client list includes the U.S. Federal Aviation Administration and the U.S. Department of Energy. But none of that really matters; the wind doesn't care about credentials.

 

Concerning the aerodynamic drag of road vehicles, I highly recommend S.F. Hoerner's Fluid Dynamic Drag (1965), which devotes an entire chapter to land-based vehicles. Want to know the drag coefficient of a freight train? It's in there.