wind pressure when driving a car on freeway.?

Question

I am dring a car at constant speed, say 65 mph, on a freeway.
What Is the wind pressure on the car if
a. I close all the car windows.
b. I open one car window
c. I open all car windows.
I am trying to use as little gas as possible when driving .
Please explain or prove your answer. Thank you.

Answer 1

Modern cars are designed to enhance laminar flow of air over the body. This results because eddy's, the nonlaminar swirls of air as a body cuts through air, are signs of energy loss. And energy loss cuts into efficiency and that milage you are looking for.

Drag forces are D = 1/2 rho Cd A v^2; where rho is air density, Cd is the coefficient of drag, A is the cross sectional area, and v is the velocity of air over the body (e.g., your 65 mph forward so the air is moving 65 mph backward over the car).

Clearly, in your problem, rho, A, and v are constant. That is, the air density hasn't changed by opening and closing windows. Nor has A, the shape and area of the car looking straight at its grill and windshield, changed. And v = 65 mph, constant as well. So what about Cd, the coefficient of drag.

Cd is in fact a measured number found by experiments (e.g., in wind tunnels). And the more laminar flow is maintained over a range of velocities, the lower Cd is. For example, a bullet, designed to knife through air at high speeds would have a very low Cd because air flow would remain laminar at those high speeds.

You need to take it on faith that Cd(closed)
With higher drag forces, it will require more force from your drive train/engine to counter act those higher drag forces to keep going at 65 mph. And that means you are pushing the gas pedal a bit further in than you'd otherwise need with the windows closed. And, voila, there goes your milage.

Answer 2

Opening the windows will increase drag. In fact, it taks less gas to run your air conditioner than to open the windows.

P.S.: I see that bluecuriosity disagrees with me, so I did a quick search. Here's some good advice from cars.com:

"Open Windows or Air Conditioning?
This is an age-old conundrum. (Unlike a car's heater, which uses free engine heat to warm the cabin, the air conditioner robs engine power and lowers fuel economy.) So which approach is better? Sorry, but it's not as simple as one or the other.

If your car has been sitting in the sun and is hotter than the outside air, drive for a few minutes with the windows open to cool it off. Then, if you're hitting the highway, close 'em up and turn on the A/C. Aerodynamics are more important at high speeds, so if you're not exceeding 35 or 40 mph, open windows won't make as much difference. It also depends on the vehicle. The detriment from driving with the windows down is greater, say, in a Chevy Corvette, which has excellent aerodynamics, than in a Hummer, which has ... none. The same applies to convertibles; you'll burn less fuel with the top up."

Answer 3

The increase in drag on your car from opening the windows is nearly insignificant. The airflow past the side of your car may or may not develop turbulence, which is what increases drag. At 65 the airflow will probably remain laminar, meaning no increase in drag.

You might notice, however, that air resistance is proportional to the square of velocity, so power lost to air resistance is also proportional to the square of velocity.
L(64)/L(65) = 64^2/65^2 = 0.96947, or 96.9%, so you can reduce the gas consumed to overcome air resistance by about 3% just by driving 1 mph slower. (Don't look for a 3% increase in gas mileage, though. Wind resistance is only one of the factors in gas consumption.)

Answer 4

the aerodynamics of your car is at its maximum with the windows up and closed.

However, if its hot out and you roll your windows up just to
turn on the AC you are expending more fuel then if you could
stand it to have the windows down.

The best way to save dollars on mileage is to manage your
accleration and deccleration, the RPM of your engine is in
pretty close proportion to the amount of gas you use.

Doing things like dropping your car into neutral on downhill
slopes will save about 15% if you drive an automatic and live
in a hilly area. This is extra wear on your transmission though.