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Old 04-02-2012, 11:47 AM   #11 (permalink)
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Quote:
Originally Posted by Sven7 View Post
Unless it's really gusting I don't really pay attention under the logic (?) that the MPG gained with a tailwind will be lost with a headwind, averaging the entire trip anyway. Is this true? If there is a 20mph wind going south and I drive north to the store at 50mph and back south at 50mph, wouldn't it be roughly the same average MPG as going 40mph up and 60mph back?
Assuming a no wind situation, absolutely not. As speed increases, so does the resistance produced by wind. This can be calculated if you know the drag coefficient and frontal area of your car. This, along with gear ratio, engine displacement, and RPM can be used to determine the absolute most efficient speed that you should travel. The average accross the United States is 55MPH, which is why it has been proposed to once again lower the speed limit on interstates to 55 - to save gas as a nation. However, there are those small cars, like a honda civic, or toyota corolla, that have a maximum efficiency at 67MPH, and the Ram 3500's that should be going 35. This broad range equals 55, but is not compatible with everyone.
The point is, that 40mph may be too low for you, and you are losing efficiency due to rolling resistance, but 60 may be too high, and efficency is lost in air resistance. Unless you have the specific numbers, a little bit of common sense can be used, but not to the point of 100% accuracy. 50MPH both ways is most likely the best route, but 40, then 60 is not the same at all.

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Old 04-02-2012, 11:53 AM   #12 (permalink)
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Quote:
Originally Posted by larrybuck View Post
I always slow my cruise speed down to compensate for strong headwinds.

I haven't ever had the opportunity to do it, living in a relatively light wind area, United States wise; but I would think that for a strong headwind, if you have a tach. in your vehicle, and slow down to match your normal un-windy cruising rpm's; that as long
as your speed wasn't too slow in that you might be lugging, that action would
help you to dial in windy day solutions!
I take it you don't have a tach... and have an automatic with like 8 gears...
When you stomp on the gas pedal, if you are in gear, the RPM doesn't just fly up, it is proportional to the speed you are traveling, as determined by the gear ratio relative to the speed of the axle. Traveling at 55 on a windy day, and 65 on a light wind day will have wildly different RPM readings, for me, this is a spread of 400RPM. So you cannot just hold the RPM steady. This would be different if you had an automatic transmission with an excessive amount of gears, or a CVT, but if you have those, odds are you are so rich that you dont care about gas, or do for environmental reasons, in which case, you think you have done your part already with your prius or leaf.
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Old 04-07-2012, 01:24 PM   #13 (permalink)
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Other factors and observations

The wind speed here can be steady 10-20 mph with all day intermittent gusts to 40 to 45 mph. Check out the National Weather Service, Grove, OK.,or 74354. Spring and Summer usually out of due South-Southwest. Winter mostly Northwest to due North.
I drive a due South and due North trip nearly every day round-trip. At 55 mph I get my poorest mpg either way in even calm air(rare).
The FIRST factor is the BSFC. I have a 1999 Mazda B-2500 2.5L 5 speed (Ranger clone). BSFC range is 2000-2400 rpm. 55 mpg drops me out of this range. MPG via Scangauge plummets to 22-24 mpg. 10-15 mph headwind at 55 mph is 18-22.
The SECOND factor is temperature. Both outside temp and engine coolant temp. If I get into the 2200-2400 range with no wind, then 34-38 mpg(outdoor summer temp's 80-100 degrees, water temp's 198). Same conditions with winter outdoor temps( 0-50 degrees) then 22-26 mpgs with a fairly air-tight full grill block. During winter outdoor temps, the thermostat doesn't even open at it's 191 degree threshold(coolant hovers between 174-184).
The best mpg's are summer outside temps and 194-200 degree coolant temp with tailwind. 10-20 mph tailwinds can result in 38-44 mpg's.
Hailstorms and severe cloud to ground lightning result in 0 mph and 0 mpg under the nearest roof or overhang and wishing I had a lower deductible.

The above is influenced by some extensive front to rear bumper aero-modding and a cab level bed topper. Will post some pic's when I have enough post's.

Last edited by landsailor; 04-07-2012 at 01:26 PM.. Reason: wrong word
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Old 04-07-2012, 05:50 PM   #14 (permalink)
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FWIW, the National Weather Service provides an hour-by-hour forecast for the next two days. Go to NOAA's National Weather Service and type in your area code. Go down to the bottom right box and click "Hourly weather graph". It'll give you something like this. Check out the purple! I love the NWS.

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Old 04-07-2012, 07:41 PM   #15 (permalink)
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Excellent NOAA site.....

Did not have this site. Thanks. Had been using wunderground.com for predictions. The NOAA site many more data points and good 'ole standard wind symbols.
As one can see, tomorrow's purple and gust prediction data points show an average day for this time of year.

Your car's name reminds me of the divorce process with the last ex.

Will dig for some pic's. The truck, not the ex.

Last edited by landsailor; 04-07-2012 at 07:42 PM.. Reason: Add a line
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Old 04-13-2012, 08:17 PM   #16 (permalink)
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The wind in Portland is almost always blowing from the South in the morning and slowly changes from the West by afternoon.

I can see up to 3mpg improvement during my morning commutes northbound. Wind is not insignificant unless it is an insignificant wind.
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Old 04-13-2012, 11:55 PM   #17 (permalink)
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I have done almost all of my long travels going to visit my parents.
The route must be in an area where there is a depression in the land, because the wind always blows from the side of the car ( looking at the motion of trees and flags )
I had installed some low slung side skirts much like BasJoos has on his Civic, but began to wonder if I was actually increasing my drag due to the huge amount of frontal area I was adding to the side of the car.
Id love it if I could get some advice on this.
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Old 04-14-2012, 12:29 AM   #18 (permalink)
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That's a tricky question Cd, i'll five you my theory on it. In 90 percent of windy conditions i would say the side-skirts would aid in reducing the drag co-efficiency. In all but a strong storm like side wind. In that case it would aid you in getting blown off the road.
The forward motion should be your primary concern as the winds will be 30 to 60 mph every time you hit the road.
I will assume that you would be using a front spoiler.
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Old 04-14-2012, 04:57 PM   #19 (permalink)
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When driving in windy conditions, I'll adjust my speed for the airspeed, not the ground speed, since the airspeed is what the aerodynamics of your car is seeing. I've thought at times about installing an airspeed indicator from an ultralight in my car to make it easier to make these speed adjustments. If you are into drafting (I'm not, since drafting has little effect on my car's mileage at normal highway speeds), an airspeed indicator would also tell you where the best position for drafting is.

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