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Old 11-28-2007, 06:09 AM   #1
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Speed and drag

I was listening to Click and Clack and they had a caller that wanted to know more about drag and FE. According to them, the greater the speed the more drag on a vehicle. That makes sense to me. Then they said that 40mph is the best speed because drag is not that big a factor at that speed. What I want to know is if drag is an exponential phenomena... does it increase exponentially with speed or is it a straight line inverse relationship? If it is exponential, at what point does speed yield diminishing returns? When it comes to aerodynamics I know only what I've read here. I know that someone here can school me a bit on the subject!
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Old 11-28-2007, 07:30 AM   #2
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IIRC drag is relative to both V^2 and V^3....it depends on what the flow around the car is like (turbulence changes things). I believe lower speeds it's V^2, and higher/turbulent speeds it's V^3.
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Old 11-28-2007, 07:42 AM   #3
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I also listen to Click and Clack on public radio.

Of course the amount of air drag is different for all cars but the way it varies with speed is pretty consistent. Short version: "Speed Kills" your fuel economy.

Drag increases exponentially with speed. I understand that the hp needed to overcome drag goes up with the CUBE of the change in speed. But there's something else that goes up with the square of the speed change - I think it's the actual amount of air resistance. Someone please chime in with a bit better detail - clencher?? Anyway, increasing speed even 10% reduces your mpg noticeably.

On the highway, if I'm not pressed for time, I'll usually drive the slowest speed the law and the traffic will allow. Often this ends up 50-55 mph. At highway speeds even a 5 mph change in speed will change your FE. Driving 70-75 is a real killer. Search and read in "Experiments" and maybe in other forums here also - there have been posts by people with ScanGauge who have posted the mpg results of testing FE at different speeds.

Somewhere I saw a suggestion that the optimum steady state speed for any car is the lowest speed at which it can be driven in the highest gear without lugging the engine or other issues. In my car this turns out to be about 40 mph but of course all cars are different. Obviously it's not a rule that's guaranteed to be perfect in all situations but you get the concept.

Anyway, air drag comes into play well below highway speeds (maybe 30-40 mph??) so it's worth paying attention to unless all your driving is on slow-speed city streets.
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Old 11-28-2007, 08:06 AM   #4
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Force increases with the square - takes 4 times more force to push something through the air at twice the speed. This is FORCE and not energy or HP since HP is force over distance it changes a bit differently.

The most efficient operating point of a vehicle depends upon many factors like Drag and also engine efficiency at different power levels. There is a certain amount of energy being used to operate the vehicle no matter what the speed i.e. lights, pumps, ignition, fans, radio, oil pump, cam shaft, engine friction, etc. So the faster you go the less percentage of the total energy is used to operate these devices and more of the total energy is used to move the vehicle. The only real way to tell what the best speed is to get the best mileage is to actually measure it with something like a ScanGauge type device.
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Old 11-28-2007, 08:57 AM   #5
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As already said...

Drag Force increases with velocity squared

Power necessary to maintain constant speed increases with velocity cubed

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40mph is really arbitrary in my opinion - I mean, I can get ~53mpg if all I did was keep my car at idle speed in 5th gear... Which is about 18mph
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Old 11-28-2007, 11:23 AM   #6
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40mph just happens to be where most torque converters lock up (when warm) in the tallest gear based on what I've read.
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Old 11-28-2007, 05:23 PM   #7
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I like 30-35 MPH in 5th gear. I used to drive a bypass highway to work at 55-65, but now I prefer going through town -- even with stoplights -- because I get better FE.
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Old 11-28-2007, 05:46 PM   #8
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Boy I't tell ya going 40 on my electric scooter . . . you would have a hard time convincing me that it was an efficient speed . . . man that wind was fierce!
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Old 11-28-2007, 06:17 PM   #9
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pure electrics are a whole different realm because the lack of transmission/gears (in most cases anyway) and very different torque curve
as for gas engines, yes as slow as you can in the highest gear without lugging the engine. for autos: pay attention to the torque converter lockup
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