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Old 03-05-2007, 12:29 PM   #1
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Question Best technique when driving in strong head/tailwinds

Today, I was driving on the highway from Regina to Saskatoon and got a lovely 70+ mpg @ 50mph when the crazy wind was tailing me , but when I turned around heading back to the city, the same speed returned 34mpg .

So then I wondered:

If I wanted to average 50mph on the highway both ways, what would be the best driving technique in strong winds? Doing 50mph both ways? or say 40mph with tail wind and 60mph with headwind? Or vice-versa? Or would it make any overall mpg difference at all?

I'd test it myself, but the wind is always changing on the prairies. My gut feeling is that if my car is going to get crappy FE in the wind, I may as well cruise at a higher speed at get to my destination. Whereas if there's a nice tail wind, I'll slow down a couple mph and reap the great FE.
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Old 03-05-2007, 12:45 PM   #2
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Originally Posted by Peakster View Post
Today, I was driving on the highway from Regina to Saskatoon and got a lovely 70+ mpg @ 50mph when the crazy wind was tailing me , but when I turned around heading back to the city, the same speed returned 34mpg .

So then I wondered:

If I wanted to average 50mph on the highway both ways, what would be the best driving technique in strong winds? Doing 50mph both ways? or say 40mph with tail wind and 60mph with headwind? Or vice-versa? Or would it make any overall mpg difference at all?

I'd test it myself, but the wind is always changing on the prairies. My gut feeling is that if my car is going to get crappy FE in the wind, I may as well cruise at a higher speed at get to my destination. Whereas if there's a nice tail wind, I'll slow down a couple mph and reap the great FE.

The speed that gives you the best MPG that traffic will allow.
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Old 03-05-2007, 12:57 PM   #3
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I hate driving into the wind but on my car the slower you go into the wind the better the mpg.
You should be able to cruise at higher speeds with the wind and still get good mpg. So I would vote for the 60/40 speed, at 60 you should get almost the same mpg with the wind as at 50. And at 40 your mpg should be a lot higher than 50 against the wind.
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Old 03-05-2007, 02:03 PM   #4
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Same as always, as slow as possible in top gear. Sometimes, I'll get lucky and can duck behind a semi into ~20-30mph winds, and take advantage of them on the way back, but it's rare.
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Old 03-05-2007, 02:32 PM   #5
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I've been dealing with high winds lately too. This is how I've been dealing for my commute; If i am going to have a tail wind, I will take the highway like normal. If I'm going to encounter a strong head wind, I take rural roads. That way I can keep my speed lower and these rural roads tend to be more tree lined, and the trees are closer to the road. These conditions tend to help cut the wind forces on the front of my car .
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Old 03-05-2007, 04:58 PM   #6
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Wind resistance increases exponentially with speed, so it would make sense to go faster with a tailwind (you will not be penalized by the higher speed as much), and slower with a headwind (yes as slow as possible in top gear).
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Old 03-05-2007, 05:51 PM   #7
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Originally Posted by Peakster View Post
If I wanted to average 50mph on the highway both ways, what would be the best driving technique in strong winds? Doing 50mph both ways? or say 40mph with tail wind and 60mph with headwind?
The 40/60 will not average to 50. For the sake of argument (and easier math) make the one way distance 60 miles. It will take 1.5 hours out at 40 and 1 hour back at 60 for 2.5 hours total for the 120 miles, an average of 48 mph.
To average some desired speed at some speed out and a different speed back it is necessary to use ratios. At 40 mph out (4/5 of 50 mph) the speed back will need to be 62.5 (5/4 of 50 mph) to average 50 mph. 62.5 mph is 1.5625 times faster than 40 mph. The air drag increase will be that speed increase squared or 2.44 times the air drag.
A 10 mph tail wind at 62.5 will make the "air speed" 52.5. A 10 mph headwind at 40 mph will make an "air speed" of 50 mph, almost an equal trade in air drag.
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Old 03-05-2007, 06:03 PM   #8
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The 40/60 will not average to 50. For the sake of argument (and easier math) make the one way distance 60 miles. It will take 1.5 hours out at 40 and 1 hour back at 60 for 2.5 hours total for the 120 miles, an average of 48 mph.
To average some desired speed at some speed out and a different speed back it is necessary to use ratios. At 40 mph out (4/5 of 50 mph) the speed back will need to be 62.5 (5/4 of 50 mph) to average 50 mph. 62.5 mph is 1.5625 times faster than 40 mph. The air drag increase will be that speed increase squared or 2.44 times the air drag.
A 10 mph tail wind at 62.5 will make the "air speed" 52.5. A 10 mph headwind at 40 mph will make an "air speed" of 50 mph, almost an equal trade in air drag.
Very good information! Thanks a bunch!

Edit: I'm not that great at math problems, so I'm going to throw this one out there:
Today I went on a flat empty highway that goes SE/NW. Temperature was -15*C with SE winds @ 41km/h.

Going SE, I accelerated like a bat out of hell to 100km/h within 0.4kms. Let's say I got 15mpg for that 0.4km stretch. I then shut off the engine and was able to coast to a complete stop in 0.7kms.

I then turned around, heading NW and once again accelerated like a BOH to 100km/h. This time I only needed 0.3km to reach 100km/h and lets say I got the same 15mpg within the 0.3km. I then shut off the engine and was amazed that I coasted to a stop after 1.6kms.

Question is this: What was my MPG(US) in each direction and what was my combined MPG(US) for the 'round trip (not including the fuel used to turn the car around)?

Edit #2 - This the the answer I came up with:
Going into wind: 62.7ml of fuel used, 41.25 mpg for the 1.1km total
Going away from wind: 47.0ml of fuel used, 95.00 mpg for the 1.9km total
Both trips: 109.7 ml used for 3.0km = 64.29 mpg
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Old 03-05-2007, 06:32 PM   #9
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I'm with ombwtfbyobbq, who wrote "same as always, as slow as possible in top gear."

But if you want to have a higher speed and a lower one to optimize things at an average speed, drive slower against the wind and faster when you're with the wind. Because the "wind resistance penalty" goes up much faster than your speed does. When the wind is with you, you are spared that extra load on the engine, which is more than you might guess at first.

The resistance actually goes up with the cube of the speed change. What that means in practical terms is that the penalty for increased speed is considerable. Here are some calculations using 40 mph as a base value.

40 mph: drag = base value
50 mph: drag = base x 1.953
Which means that the air resistance that you are fighting at 50 mph is essentially DOUBLE what you had at 40 mph. But your speed is only increased x 1.25, so you're really losing ground on that score. You are creating your own resistance that you have to fight against.

60 mph: drag = base x 3.375 but your speed is only x 1.5 the base speed
70 mph: Drag = base x 5.359
80 mph: drag = base x 8.000 but your speed is only x 2 the base speed

So if you needed 5 hp to overcome wind resistance at 40 mph, you need 5 hp x 8 = 40 hp to overcome it at 80 mph. Those 40 hp will cost you a lot of fuel.

Of course a more aerodynamic body like an Insight will give you less resistance at any given speed, but whatever resistance there is goes up by the same cube-based percentage.
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Old 03-05-2007, 07:45 PM   #10
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Originally Posted by James View Post
Wind resistance increases exponentially with speed, so it would make sense to go faster with a tailwind (you will not be penalized by the higher speed as much), and slower with a headwind (yes as slow as possible in top gear).
If your tailwind is high enough, you can open the doors and drive "Wing-n-wing."

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