How much does wind play into MPG?

[WutGas?]

First, let me say that this was just a little experiment I did. It actually means nothing but I found it interesting none the less.

So the other day, it was very windy in OKC (even by our standards). We were having 30-40mph winds blowing straight north. I thought it would be fun to see what difference it made in my commute. I had to run to my casa from a friends house. It is about 15 miles straight north and south. I make this trip often and the variations in wind or elevation usually don't make much if any difference.

Since it was super windy, I thought I would start my MPG 1 on the MFD when I turned onto the highway, and see where it landed, and then do the same thing on the way back since I was doing them back to back.

Drove the 15 miles directly into the wind and the MPG calculator showed 38.9 when I exited. This was with the cruise set at 62mph.

On the return trip, about 10 minutes later, I did the same thing, but with the winds pushing me. I went the same 62mph and when I exited, the MFD was showing 68.6 mpg.

I know, I know, it's just a small sample size and doesn't mean much, but I still thought it was pretty amazing that it was that much difference.

 

[JSWTDI09]

Wind speed (head wind or tail wind) has a similar effect as actual driving speed. As any pilot knows air speed and ground speed are usually different depending on winds. The higher the "air speed" the more wind resistance you will see. Wind resistance is the main cause of mileage decrease with speed. It is not the only factor, but it is probably the main factor. This is why aerodynamic body shapes (and modifications) increase mileage in cars (drag coefficients).

You will always get better mileage with a tail wind and worst mileage when driving into a head wind. The faster your "air speed" the more aerodynamic drag your car will encounter. Aerodynamic drag is the biggest factor in speed related mileage drops. There are also mechanical inefficiencies and friction losses, but aerodynamic drag is a big factor.

If you are driving at 60mph into a 20 mph headwind your "airspeed" is 80mph. If you are driving at 60mph with a 20mph tail wind your airspeed is only 40mph. This can make a huge difference in your mileage.

Have Fun!

Don

 

[Ski in NC]

Also, crosswinds screw up mpg, though not to the same effect. So if you were going east or west, you would have taken a mpg hit. Doing vector math shows that the crosswind does add to the vehicle airspeed. Also, cars are designed to be aerodynamic with the wind head on, or at least I think they do.

So on a windy day, you only have about a 25% chance that it will help you.

More useless tidbits: If you put car in neutral, that 40mph wind might have pushed you home at say 20mph and that would get you some serious mpg!!

You guys need to build some more wind turbines out there in OK!!!

 

[nicklockard]

Also, crosswinds screw up mpg, though not to the same effect. So if you were going east or west, you would have taken a mpg hit. Doing vector math shows that the crosswind does add to the vehicle airspeed. Also, cars are designed to be aerodynamic with the wind head on, or at least I think they do.

So on a windy day, you only have about a 25% chance that it will help you.

More useless tidbits: If you put car in neutral, that 40mph wind might have pushed you home at say 20mph and that would get you some serious mpg!!

You guys need to build some more wind turbines out there in OK!!!

That's mostly right. Winds from the front right or left quadrants reduce FE the worst, versus from straight on. For just the reasons you put: the car is designed to go forward. From an oblique angle, the frontal area is MUCH higher, 2-3 times higher, and the Cd I'd estimate around .6-.8 for most sedans, which look like a block of wood from the side.

Plus, tire rolling resistance increases due to the sideways component of the force vector: you have to keep the wheel into the wind to stay straight, and that means lots of tire scrub.

Conversely, a helping wind from the rear obliques should help the most. Ernie Rogers has posted more detailed analysis on this topic. He's an ex aerodynamic engineer that used to post more frequently.

 

[WutGas?]

You guys need to build some more wind turbines out there in OK!!!

That's no bs. I think there are a few around here somewhere, but not many. Oklahoma, as a whole, isn't the type of folks that care about alternative energy unfortunately. Too bad I got stuck here. Maybe one of these days after my daughter is grown I will nut up and move somewhere else.

 

[deezelpower]

With my scangauge, I've witnessed my highway MPG at 75 mph go from 42 > 65+ when someone is in front of me. I followed someone for a half hour or so and averaged 67 mpg.

I would guesstimate the wind drag is >90% of the cause of mileage loss at high speeds.

 

[migbro]

That's mostly right. Winds from the front right or left quadrants reduce FE the worst, versus from straight on. For just the reasons you put: the car is designed to go forward. From an oblique angle, the frontal area is MUCH higher, 2-3 times higher, and the Cd I'd estimate around .6-.8 for most sedans, which look like a block of wood from the side.

And that would be totally wrong. The worst wind direction for FE is a direct headwind.

You'd be right if the car was traveling sideways down the highway. But it isn't. Its longitudinal axis is always parallel to its direction of travel and the quartering headwind you describe can be resolved into a headwind component and a crosswind component. The effective frontal area does not change and with a quartering headwind the headwind component is less than it would be if the wind was a direct headwind.

There is a secondary effect in that the crosswind component disrupts the airflow over the car which may increase the drag coefficient a little.

Plus, tire rolling resistance increases due to the sideways component of the force vector: you have to keep the wheel into the wind to stay straight, and that means lots of tire scrub.

Got that one right.

Conversely, a helping wind from the rear obliques should help the most.

Wrong again.

Sounds like Ernie made a basic mistake. He drew his wind vector diagram but then forgot that the resultant wind vector and vehicle direction of travel are not reciprocal.

 

[nicklockard]

http://ieeexplore.ieee.org/xpl/logi...re.ieee.org/xpls/abs_all.jsp?arnumber=5366915
http://calteches.library.caltech.edu/528/2/Bettes.pdf
http://www.ecoflaps.com/subcat.cfm?MainCat=68
http://forums.tdiclub.com/showthread.php?p=1097194&highlight=crosswinds#post1097194
http://forums.tdiclub.com/showthread.php?p=1094125&highlight=crosswind#post1094125
http://forums.tdiclub.com/showthread.php?p=2763153&highlight=crosswinds+effect#post2763153
http://forums.tdiclub.com/showthread.php?p=2734115&highlight=crosswinds+drag#post2734115
http://forums.tdiclub.com/showthread.php?p=1571723&highlight=crosswinds+drag#post1571723


The topic is more complex than I presented it, and I didn't do it justice, migbro. Effective Cd is a vector dot product of static Cd and V; same is true of cross section. Sort of...it depends on whether you're tallying pressure drag (which mostly comes from the failure of the air to reattach at the rear of the car, which is a suction...this is ~70% of total aero drag on a vehicle) or whether you're talking skin drag (where static Cd and static A are all that matters).

In simple terms (but I'm sure still won't do it real justice...bear with me), pressure drag is worse for slight angles of headwinds, and this forces you to hold the wheel to keep the car straight. The increased tire scrub and pressure drag drops FE.

You can experimentally confirm or disprove this easily: use a smartphone to determine wind directions from NOAA.gov or similar respectable weather website. Go on a longish drive on a nice straight road with scangauge. Compare FE for different relative angles of winds on different days. You'll have to have steady winds like the Santa Anna's to get the best data.

People have done this. I'm right, but explained it poorly. I don't have enough time, or I'd do a better job.

 

[DonL]

Back about 1983 I drove my diesel Dasher wagen from near Seattle to Long Beach, CA. There was a significant tail wind blowing down the Central Valley. I filled up in Corning and filled again at Castiac. At Castiac I filled it until I could see the diesel in the filler neck. It came to 67 mpg. The actual was higher since I filled it more than the previous fillup. Normal for that car at my driving on the highway in the days of 55 mph was 48 mpg.

In October we drove up to Oregon for my son's wedding. We had a tail wind going both ways. That does not happen often.

Not only does a wind make a difference, how fast you go into it or from it makes a difference too.

 

[migbro]

Effective cross section and Cd most certainly increase when heading into an oblique angle wind versus straight on. And that results in you having to hold the tire over to keep straight. Both factors combine to give worst FE.

By that logic the worst FE would be with a 90 degree crosswind as, by that logic, the effective cross section is at a maximum. You need to think about this a bit more.

The key to understanding this is to sketch out some vector diagrams with the car's direction of travel, the wind direction and the resultant wind vector. With a quartering headwind the resultant wind vector will be at an angle to the car's centerline, thus leading to the conclusion that the car's effective cross section is increased. But that conclusion is wrong. It would be true only if the car's direction of travel were reciprocal to the resultant wind vector. But it isn't. As the car's direction of travel is different the resultant wind vector should be separated into a headwind component and a crosswind component. The headwind component acts on the car's frontal area and requires engine power to overcome. The crosswind component requires a steering offset to overcome and will thus reduce FE slightly, as you say, through tire scrubbing and perhaps also by slightly increasing the car's effective drag coefficient.

 

[WutGas?]

I knew this would turn all scientific...lol.

 

[nicklockard]

By that logic the worst FE would be with a 90 degree crosswind as, by that logic, the effective cross section is at a maximum. You need to think about this a bit more.

Not true. You're making a straw man argument. I never said the relationship was linear, as your statement here implies. It's a complex relationship of several variables, more important to suction drag, and how that affects your steering.

The key to understanding this is to sketch out some vector diagrams with the car's direction of travel, the wind direction and the resultant wind vector. With a quartering headwind the resultant wind vector will be at an angle to the car's centerline, thus leading to the conclusion that the car's effective cross section is increased. But that conclusion is wrong. It would be true only if the car's direction of travel were reciprocal to the resultant wind vector. But it isn't. As the car's direction of travel is different the resultant wind vector should be separated into a headwind component and a crosswind component. The headwind component acts on the car's frontal area and requires engine power to overcome. The crosswind component requires a steering offset to overcome and will thus reduce FE slightly, as you say, through tire scrubbing and perhaps also by slightly increasing the car's effective drag coefficient.

It's more complex than that. Take some time to look at the CFD results. As I said, I didn't have adequate time to present it well. I'll try and back-fill as I get time.

A condescending attitude and dismissive writing tone don't make your arguments any more effectively.

The vector component of the wind at an oblique angle is LESS than one head-on.

No one is arguing otherwise. BUT, pressure drag can go up because the increase in dynamic Cd and dynamic A can overcome the decrease in v^2 at some angles of attack (different for each car). You're also forgetting or ignoring interference effects to flow reattachment patterns from wheels, wheel wells, and underbody drag.

Your argument is predicated on static Cd and static A as calculated in a wind tunnel with no rolling road effects and an 'all other things are equal' perspective. Mine is predicated on real world and dynamics--all other things are NOT equal. As I said, mine is testable, with simple tools available to anyone. Yours is textbook with angry delivery.

 

[nicklockard]

I knew this would turn all scientific...lol.

I'm sorry man.

For what it's worth, I've experienced my worst mileage in gusting winds on rainy days. The constant gusting forces me to have to correct the wheel to keep the pointy side forward, and that eats momentum. The rain sticks to the tires and increases rolling resistance. I went out to Hood River and back this weekend and lost 13% FE from usual on the round trip. Normally I'd have made back what I lost on one of the legs and they'd mostly cancel out. But the gusting and shearing rain was pretty bad.

 

[migbro]

Your argument is predicated on static Cd and static A as calculated in a wind tunnel with no rolling road effects and an 'all other things are equal' perspective. Mine is predicated on real world and dynamics--all other things are NOT equal. As I said, mine is testable, with simple tools available to anyone. Yours is textbook with angry delivery.

So, your argument is essentially that the answer is too complicated to understand without the use of computational fluid dynamics.

I disagree. A resultant wind vector that's a quartering headwind will produce much higher total drag on the car but the total drag does not have to be overcome by power produced by the engine. Much of it is overcome by the friction force exerted by the roadway on the tires.

You can often get good insight into these kinds of problems by doing thought experiments at limit conditions. Here's one.

The car is stationary, engine idling, brakes off. There is a 100 mph direct crosswind exerting a very large drag force on the car. How much engine power is required to overcome this drag force?

One more question. If your approach is correct, then using CFD you should be able to calculate a critical angle at which FE is lowest. This critical angle would be a function of total aerodynamic drag and headwind component. What is the critical angle? An approximate answer would be fine. Please make any assumptions you feel are necessary to provide an answer.

 

[WutGas?]

I'm sorry man.

No need to be I actually enjoy it. You guys know way more about that stuff than I do. The only thing I know is that :in my best cave man voice: "head wind....BAD! Tail wind....GOOD!"

 

[Rickstah]

That's no bs. I think there are a few around here somewhere, but not many. Oklahoma, as a whole, isn't the type of folks that care about alternative energy unfortunately. Too bad I got stuck here. Maybe one of these days after my daughter is grown I will nut up and move somewhere else.

Wut, you cut us to the quick, man!

Oklahoma is #6 on the top ten wind energy producers in the country, and we have at least 17 windfarms here. I have 100% of my electric bill credited to wind generated electricity, I know many who do, also.

That said, I did the same thing the other day when you did, although Christine doesn't have a computer. I was able to coast almost a mile up Mustang road at 20+ mph, lol...deserted section of course .

 

[nicklockard]

migbro, let us use our visualization skills here, and it will become apparent.

I'm borrowing Figure 1.5 from this paper: http://dumas.perso.math.cnrs.fr/Cha1.pdf

First off, let us establish: can you agree this is a pretty good depiction of turbulent flow, for the real world?

Next, imagine headwind of 10 mph at 20 degrees off the bow.

And now, using visualization skills, imagine how the shape of the departure envelope changes. Can you agree that some of the wind will depart from laminar flow into turbulent flow at the rear-quarter sides of the vehicle? I hope you can see or visualize that for *some* car's body shapes at *some* speeds where Vwind is a significant fraction of Vforward. Let's look at normal wake flow departure cross-section:

Figure 2: wakeflow departure cross section from head-on, 10 mph wind

And now let's look at crosswind wake flow departure cross section:

Figure 3: wakeflow departure cross section from 20 degree, 10 mph crosswind. Here I've marked an 'X' where the wake flow of the left mirror is interfering with the rear of the car's wake flow pattern.

The shape of the wake flow departure cross section has both GROWN in area and shifted in shape.

THAT is the effective cross section that changes. The phrase 'frontal area' is unfortunate because it STRICTLY relates to skin friction and the 'size of the hole you punch through the wind' which is what you are hung up on. What is important to wake drag (pressure drag) is the effective cross section where the flow departs from the car, NOT 'frontal area'.

If you can visualize THAT departure cross sectional area, you can now also visualize how Cd changes (it's a different apparent shape to the wind now) AND you should be able to extend to see the effects of interferences between wake flow from tires, underbody parts, and mirrors. Where before those turbulent zones 'stayed in their swim lanes' mostly, you should easily visualize how they will now spoil the flow of other laminar regions. The interference effects won't be steady-state flow patterns, but are most likely to be a 'buffeting vortices'.

It has a deleterious effect on wake drag. Wake drag is ~70% of total aerodynamic drag.

And, we haven't even considered the steering angle change and increase to tire scrub and rolling resistance.

This is testable, by anyone with a scanguage and knowledge of wind velocity (speed, angle) in a steady wind.

 

[Jbdesigns]

and do you know what sucks, driving the commute with a head wind, both ways!!
that has happened on occasion. early morning wind direction is mostly headwind but by afternoon, the front or weather pattern has changed and the wind direction opposite. Damn!

with our cars getting such good MPG ratings, a hit from headwind, or cold weather really shows up more. maybe it is only a few % worse, but that adds up to a larger diff on the MPG gauge. someone achieving only modest MPG # of 20 may not hardly notice the hit or think it was big. 20 goes down to 18, and they might think, no biggie. but your TDI getting 50 drops to 45 and you go, *** happened?

it is all fun and why i enjoy watching the numbers on my commutes. gives me something to think about. like, why is the MPG better or worse than average, is it wind? cold weather? driving too fast? take a route with more hills? etc.

 

[RNDDUDE]

I used to live in Wyoming, one of the constintently windy places in the USA.

A popular saying back then was "It doesn't really blow in Wyoming, it sucks in Nebraska.."

 

[EddyKilowatt]

and do you know what sucks, driving the commute with a head wind, both ways!!

I live near the ocean and have this pretty routinely due to the daily cycle of morning land breeze and afternoon sea breeze. Oh well... at least there's the ocean to look at.

Most of my headwinds have a sidewind component, and my observation is that they seem to cause more drag than you'd think just based on the headwind component. I haven't been rigorous about measuring it, but my experience seems to support Nick's hypothesis and analysis.