Windows down on VW Golf = 2.5% FE reduction

[MetroMPG]

Originally posted: 08-03-2006, 09:46 PM

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This could just as easily be a thread about "windows down vs. air conditioning" for best efficiency, but I'm more interested in the aerodynamic implications of the info collected:



Source: Energy Efficient Transportation for Florida

(I don't have a/c, that's why it's not even an issue for me. )

No - the interesting part is seeing the aero impact of windows down quantified for a car with the same basic shape as the Blackflea:

Some sources claim that closing windows and using a car's air conditioner will result in higher fuel economy at freeway speeds than leaving the windows open without the air conditioner. FSEC tests showed this in not the case. In repeated evaluation at 65 miles per hour, our test car experienced 11% better fuel efficiency with no A/C and the windows open than using the air conditioner. We also found that closing windows at freeway speeds improved fuel efficiency by 2-3%.

[MetroMPG]

It doesn't say, but I'd assume windows were all the way down. And it's a North American GTI, so it only has front-opening windows.

It's about the FE penalty I would have expected to see for windows all the way open.

[MetroMPG]

I thought 2.5% was realistic based the size of an open front window compared to the size of an open wheel well. Covering my rear wheel wells improved my FE about 2.8%. "Uncovering" the GTI windows reduced its efficiency by 2.5%.

I realize they're not the same things (one doesn't have a spinning wheel inside it, further stirring up the air). But it's in the ballpark.