Pick up truck drag?
 

will opening my back window on my T-100 truck increase drag? If so, how much?

Nobody responded in the other thread you started to ask this question. I was going to but lately I've taken a hit to my confidence on technical issues like this, as I've provided a few wrong answers... :(

Anyway, here's my thoughts:

- With the window closed there is a vortex directly behind the cab. Have you ever driven with dry dirt dust, leaves, dry dusty snow, or a plastic bag unsecured back there? The air apparently rolls forward and up until it hits the rear window; you can see dust rolling in circles back there, and snow gathers on the rear window when blow up out of the bed.

- Assuming your side windows are open, you probably get lots of air flowing through, pushing on that vortex and maybe breaking it up. Perhaps it would straighten it out and make it fall off the back of the truck. I have no clue how it would affect fuel economy.

- If your side windows aren't open, that vortex is going to push into the cab, which will act like a parachute. I think this will slow the vortex but not break it up. It probably won't have a measurable effect on fuel economy.

holycow- I have always thought highly of your advice and consider it a positive addition to the site. also, it is good to be wrong every once in a while, it keeps us in check and keeps us from getting cocky. also, there are different ways of looking at everything and almost impossible for one person to look at something in all the different angles.

I too agree that the back window will make you take a hit in FE. the question is by how much. I really don't think it would be by much. also, the comfort factor. you do want to be comfortable going down the road. I usually ride with windows down, as my A/C doesn't work, regardless of the FE hit I take for it because I wan't that level of comfort.

exactly, when its 92 out with 75% humidity in bright sun, ya windows down all the way and vent on high...

i doubt havign the backwindow will have a real effect on it around town driving, interstate maybe but even then just crack it open.

My old 96 ranger had a sliding rear window and with the side windows open, air would pour in the sides and out the back...the vortex is a definite low pressure zone no doubt. I guess the question is whether it's more drag to let the air in right after the windshield or force it to go in after the cab.

with the side windows closed I doubt it'll have any appreciable difference.

My S10 doesn't have a sliding rear window so it's a moot point.

it does on my 77 chevy

my GUESS, also applies to windows down n such... how ever much air is swirling around in the passenger compartment, is wasted energy. what wattage fan would it take to create as much air movement if you were parked? 200watts, 1000, more? i think 746 watts = 1hp?
i have a 210w fan at work, i'd guess its moves air around about as much as windows down in my car at around 40mph

thanks for the response guys.
C

theholycow - I know what you mean by taking a hit. Don't let those "tailgaters" put you in the corner in the fetal position whimpering! Opinionate away. Advise as you will. For every person that might say a bad thing, there are at least 10 that hang on to every word.

That's a GOOD question on your rear slider window. That's grounds for an experiment. Also, there could be a question of both passenger and driver windows down plus the rear slider window open. How would THAT affect wind resistance passing over the cab/bed? We could do the math in our heads on that, but it really needs real-world data.

i used dry leafs in the bed test,,,,,,:D