Aerodynamic

[TRDFerguson]

Aerodynamics

I was thinking about this as I was driving down the highway with my windows down.

So the old wives tale is that driving with the windows down actually nets worse gas mileage than driving the same speed with the ac on and windows up.

My question is, does the fact that we can roll down the rear window change those aerodynamics? It seems like the drag is considerably less when you have all the windows down versus just the side windows.

Just wondering!

I don't know who will know a definitive answer... Any aeronautical engineers in the house?

 

[criera]

Theres an app for that lol

Mythbusters did the side windows, deff confirmed, ac was negligible but windows down had substantial negative effect.

Not sure about the rear, though i have heard of a vacuem effect essentially sucking from bad aero.

 

[criera]

And open

 

[4Runner4TW]

This was from the Mythbusters. I believe it was a Ford Expedition. Not sure what the rear window would do on this. Interesting though:

Windows down vs. air conditioning
"Urban puzzle": it is more efficient, on a hot day, to run with the A/C on and windows up than to run with windows down (b/c of increasing car's drag).

Computer-based mpg measurements: * 11.7/11.8 with A/C on and windows up * 11.7/11.8 with A/C off and windows up * 11.3 with A/C off and windows down

So, according to the computer, it's better to use A/C with windows up.

This was too quick and easy for TV, so they decided to stage a seven hour marathon, race-til-you're-empty duel, with Jamie driving an SUV with A/C on and Adam driving an SUV with windows down. Though, once the safety inspector intervened, it was no longer a seven-hour marathon, it was a bit slower (45mph instead of 55mph), and a lot shorter (only 5 gallons each).

Jamie's A/C car ran out of gas first -- Adam's windows down SUV ran for another 30 laps -- completely contradicting the computer mpg estimate. Computer estimate based on air flow into the engine, so it would appear that it is unable to properly model the difference between A/C and windows down.

 

[shootmymime]

kind of off topic but maybe someone else didnt know this like my buddys did.

so i was sitting in the back seat of the pick up one day driving around in the country and we were getting low on fuel but it was 100+ deg in arizona so of course the ac was on. i was burning up in the back so i told them to turn the fan up to get more air moving. they said no because it uses more gas.

their limited thinking of how the system worked was the faster the fan speed of the AC, creates worse fuel millage. but that is not the case. if the AC is on, its the drag on the engine for the compressor that you loose fuel millage not the fan speed that is run off eletricity. need less to say, i convinced them and got to ride the rest of the trip with nice blowing AC.
PS. we didnt run out of gas.

 

[White 4x4runner]

So ya wanna know about areodynamics eh? Well I can tell ya my runner wasn't good enough so I went out and bought me a Volvo 242. Now that my friend is aerodynamic.

 

[Desert36]

I've got a variable to add.... On my daily driver I was getting great gas mileage a/c-w/up.. Had to replace the ac pump from factory to refurbished garbage and the mileage took a dump because of the crappy new pumps drag on the engine...
Of corse I wanted to source a more expensive pump but the math on a $600 good one vs keeping what I got and getting less mpg would take years for it to offset.

 

[SilentYoda3]

What about the rear visor thing? Does it help or hurt mileage? Just curious.

 

[SilentYoda3]

What about the rear visor thing? Does it help or hurt mileage? Just curious.

[MENTION=56510]criera[/MENTION] any ideas?

 

[Kanger328]

What about the rear visor thing? Does it help or hurt mileage? Just curious.

I think the rear "spoiler" is to rush wind down the rear window to keep dust off. I had an SUV before with no rear spoiler and the window would always get really dusty. Could be wrong tho...

 

[snivilous]

What about the rear visor thing? Does it help or hurt mileage? Just curious.

I think the rear "spoiler" is to rush wind down the rear window to keep dust off. I had an SUV before with no rear spoiler and the window would always get really dusty. Could be wrong tho...

Not quite. The rear spoiler is meant to help mileage by redirecting air into the vacuum created behind the vehicle. If the rear window was open then the redirected air could potentially go into the cabin and be of less use, though it's probably a negligible difference if any.

Think of a block moving along, the air gets out of the way by moving to the sides, however it doesn't smoothly flow back in behind the block to where it originally was. Instead you get an eddie formed that decreases smooth airflow, so the spoiler tries to force air in and stop the eddies from forming.