OT- Apparent wind effects on vehicle mpg

Milacron · Sep 2, 2006

Was driving on I95 Thursday night in southern NC during the peak of Ernesto's little wind and rain event and noticed my F350's mpg reading was way below usual... 16.2 mpg, when normally I'd be getting 19.2 or so. I presume this was due to driving into the counterclockwise rotation of wind of Erensto just to the east.

Judging from the bend in bushes and how it felt out of the truck, I'd estimate the true wind was about 30 mph , which would give an apparent wind speed of 100 mph at the truck (presuming I was on 0 degree heading into the wind of course..when in reality it varied some), considering it was going about 70 mph.

1. Just for fun, anyone know of a formula to calculate expected mpg deterioations based on above information ?

2. Would the friction of water (i.e. pouring rain) come into play ?

Evan · Sep 2, 2006

Power required to overcome drag increases as the cube of the velocity. Also, any wind that is anywhere from 9 o'clock to 3 o'clock has a headwind component when you are moving. That's one of the things that my Jeppeson circular slide rule is used to calculate. It's a basic vector function and the apparent wind speed to be added to the vehicle speed is a sine function of the heading of the wind compared to the vehicle.

[added]

BTW, keep in mind that although the hp required goes up 8 times for a doubling in speed you get there twice as fast so the factor drops to a four times increase in fuel consumption.

The rain makes little difference. It didn't slow down my airplane noticeably.

JST · Sep 2, 2006

The drag, with or without wind speed, may or may not be the largest power consumer......

If it is, the mpg will indeed vary according to predictions based on speed and resulting drag.

But, if the major power consumer is lugging a large load up hills, then the drag contribution may be lower in comparison. It may not be the mpg determiner.

Also, with varying crosswinds, etc, the drag of tires can go way up, as they are "scrubbed" across the pavement. You have to hold the wheel slightly off of straight to go straight, and the tires are "scrubbing" the whole way. That really increases tire friction.

Water on the pavement is another one, you have to push away the water (presuming you are not hydroplaning). That takes power.

There are a lot of friction components. Drag is only a major factor when it is large in comparison to the sum of the others. Depending on speed, weather and other conditions, it may be small or large. The vehicle shape also has a big influence.

Ugly non- aerodynamic shapes like a pickup DO have far higher drag than a decent sedan, which can often be helped by adding a cover. Stick-up lights and "rollbars", "brush guads" etc have a lot of drag also.

But I don't think it is correct to focus solely on drag when there are so many other obvious and uncontrolled contributors in the situation you described.

But it is a fact that a headwind adds to your apparent speed. You can just feel that, as the vehicle slows when a stiff headwind hits, as when you come around a curve out of a wooded area.

Carl Darnell · Sep 2, 2006

I know that wind resistance and aerodynamics play an important role in fuel mileage but in real life I find it doesn't always compute. For instance I have a '93 Ford Ranger V6 3.0L. with topper and 245,000 miles on it. I reciently went from Taylorsville Ky to Conyers Ga and round trip drove about 70 mph. I got 25mpg on the trip. Does that mean that had I drove at 55mph with less wind resistance that I could have gotten 30mpg. I doubt it.

Evan · Sep 2, 2006

[quote[Does that mean that had I drove at 55mph with less wind resistance that I could have gotten 30mpg.[/quote]

That's actually a pretty reasonable assumption. Try it some time. Most vehicles get the best mileage at around 40 to 50 mph.

I have the same truck BTW.

JohnnyJohnsoninWI · Sep 2, 2006

Sometimes you get a feel for things which, at first, seem completely unrelated to the original topic.
I used to do a bit of bicycling and I would swear that it took twice as much energy output on my part to cruise at 18 mph as it did at 15 mph.
Like Evan said, Power to overcome drag increases as the cube of velocity. A 30 mph headwind would add a significant drag to your vehicle.
John

Ox · Sep 2, 2006

Don - Ever drive acrost Nebraska / Iowa?

ALWAYS a headwind heading west. For some reason however - it doesn't always seem to return the favor on the way back east?

Think Snow Eh!
Ox

Forrest Addy · Sep 2, 2006

Wind resistance has a real effect on gas milage. I had a 1980 Cadillac; great road car but not so good on milage. On last year's trip to marry off my niece I got 19.1 MPG when I drove at 50 to 60 MPH to the first gas stop in Vancouver. My niece drove the balance at 70+ MPH when we got 17.4. It takes horsepower to move air aside.

Try this sometime when a local hill is empty of traffic. Terminal speed of a vehicle coating in neutral down a hill is a function of the slope and the sum of drag components. When the coaasting speed stabilizes you're at terminal speed. Coast the hill to get a clean figure. Try it again with the door held open against wind resistance. Significant difference huh?

My bro-in-law's 2002 Lincoln has a slippery jelly bean shape within a few hundred pounds and has about the same bulk as that boxy 80 Cadillac. On the same hill he coasted about 12 MPH faster than I did. Yse, I know, retired guys don't have enough to do.

Norman Atkinson · Sep 2, 2006

My wife and I have been having this discussion.
British Motorway speeds are 70 and open non dual carriageway roads 60mph.

We are having problems and are now only getting just under 50mpg with a 2.7 liter diesel and going at 10% over these indicated speeds.

Are we doing something wrong?

Worrying times!

Norm

rke[pler · Sep 2, 2006

I was towing a trailer into a stiff headwind, something like 50-60 MPH in eastern New Mexico. I'd normally get something like 12 mpg, drove 200 miles getting only 8 mpg. Worst part was that I could hardly keep going at 40 mph, pedal was on the floor the whole time.

Evan · Sep 2, 2006

We recently took a long drive to Alberta in my PT Cruiser to visit the kids. On the way back while at my daughter's place I was studying the car and something occured to me. I have a factory roof rack and the manual recommends the both bars be slid all the way to the back on the rails. It looks a lot like a spoiler then just above the back hatch.

I spent a lot of time working on aircraft and some aircraft have parts on the wings called turbulators. The Cessna Citation jet has a fence of little angled vane shaped stubs that stick up about 1.5". They are placed about 2/3 back from the leading edge of the wing on the inner 1/3 of the wing. These reenergise the boundary layer airflow over the wing and prevent it from seperating. This reduces the drag and also makes the flaps more effective.

I decided to try this on the PT by moving the roof rack forward about 9" on the rails.

We drove back the remaining 600 kliks and I was amazed to find the mileage increased at least 10%. I had no tailwind and even some headwind so that wasn't the cause. I have also seen a small increase in local driving which is nearly all at speeds below 50 mph.

LD · Sep 2, 2006

Damn, we're not supposed to put on the old thinking cap when we reach an advanced age, but, there is a coeffecient of drag calculation (C sub D)

Drag equations depend on several factors and there is induced drag and parasitic drag:

1. Airstream velocity (V) (usually in knots but what the hell, MPH is close)
2. Airstream density ratio. (r) (yes, wet air is more dense, and heated dry air is less dense)
3. Planform area in sq ft. (S) (frontal area of your truck)
4. Profile or shape (how slippery is your truck)
5. Compressibility effects of the air.

the first two ( 1 and 2)would determne the dynamic pressure (q) of the airstream pushing against your truck. Number 4, 5, and 6 determine the amount of drag. (D)

The basic drag equation: D = C sub DqS

Coeffecient of drag (C sub D)is the ratio of the drag pressure to the dynamic pressure.

Once you find this you can multiply times your known MPG at a given speed without drag to get the expected MPG with a given headwind and density altitude.

Hmmmmm. Probably need to know tire and driveline drag.
I need more coffee

Evan · Sep 2, 2006

Once you find this you can multiply times your known MPG at a given speed without drag to get the expected MPG with a given headwind and density altitude.

You are over complicating it. Regardless of the drag coefficient of the vehicle the power required still increases as the cube of the velocity. Ignoring all other factors it will still take eight times as much power to go twice as fast. So to go 70 instead of 55 requires 2 times the power.
[edit]
That is 2 times the amount of power that was used to overcome the drag, not 2 times the power needed to make the vehicle go 55 mph.

Naturally going 70 instead of 55 doesn't reduce the gas mileage by 2 times because there are many other factors. Tire rolling friction increases non-linearly and is nearly the same at 70 as at 55. Drive train friction increases linearly at most and probably a lot less. Engine efficiency may increase depending on where it is operating in the torque curve and may actually be more efficient at 70 than at 55, or not.

[also edited for slight math slip up, 2 times, not 2.6 times]

LD · Sep 2, 2006

The question was what to expect with increased headwind and air density due to rain.

Not how much more power and gas do I use at 70 vs. 55.

Going 55 with a 25 mph headwind is not the same as driving 70 mph without wind. At the higher speed, engine RPM increases and torque increases; more hp is used to overcome parasitic loss in the drive train and road surface friction, plus induced drag from forward air compression increases. But it is the increased inhalation of fuel at the higher speed that drops the MPG most dramatically.

With just increased wind and rain (more dense air), the engine only has to overcome the increased induced drag. All else remains the same, but you need to know the rest to do the math.

Evan · Sep 2, 2006

Staying at 55 with an increased headwind may be worse than going faster depending on factors such as gearing. Although rpm won't increase engine load does and may likely place the engine in a more inefficient operating area.

Induced drag is cause by vortex generation from the ends of surfaces, not by compression of the air. At substantially subsonic velocities (less than mach 0.5) air acts as an incompressible fluid and is not compressed in front of the vehicle or anywhere else. The induced drag is included in the cube ratio for drag increase with velocity.

jim rozen · Sep 2, 2006

Car manufacturers go to *great* lengths these
days to reduce drag coeficients. Wind tunnel
testing and so on.

It's at the point where the mileage on smaller
cars will drop noticeably if you drive around
with the windows open.

I think they go so far as to remove the side
view mirrors when testing for mileage nowadays.
Seriously.

Jim

Carl Darnell · Sep 2, 2006

There is an element of driving that is offen overlooked in getting better fuel milage. The ability of the driver to maintain a speed of say 60 mph. Most, if not all, drivers will fluctuate in speed. When that happens your gas milage will be a little higher. Since the advent of speed controls that problem is somewhat lessened. I find my time is more valuable point A to B than a few mpg. so I just set on 70 and pity pat on down the road.

Norman Atkinson · Sep 2, 2006

On my MiniCoopers, I used to have a vacuum gauge siamesed onto the inlet manifold.

The answer was really to heave both the inlet and exhaust manifolds off and match them to the
gas flowed cylinder heads and to break the beaks in the heads and so on to get better breathing.

Slightly different to the topic? You must judge that one for yourselves.

Norm

Evan · Sep 2, 2006

That's a port and polish Norm, not to be confused with polish off the port.

Norman Atkinson · Sep 2, 2006

Evan,

Sorry, friend! The Rose d'Anjou is a bit potent.
I missed the Douro bit completely!
Sesimbra, Cascais and the sand blowing across the road from the Atlantic- Oh and the Portugese Grand Prix. And for those who don't need Google,
there are marble washbasins and statues in the Campsite.Wonder if the snooker table is still there?

Interesting stuff, that white aperitif sherry.

Cheers
Norm

NB Must tell about Margaretha sometime!
She and I wear the same badge on our ski suits!

OT- Apparent wind effects on vehicle mpg

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