Test: Windows open vs. closed - Fuel Economy, Hypermiling, EcoModding News and Forum

GRU · 07-10-2012, 02:37 PM

Yesterday i did a coastdown test with my 4dr Civic. I did 2 tests in each direction with all windows closed and the same test with front windows open thinking this would be worse on fuel economy because it acts as a parachute.

I used my android phone with Torque app to log the coastdown times so they are the most accurate you can get.

The result, exact same coastdown time with windows open or closed.

BamZipPow · 07-10-2012, 03:09 PM

What distance was the run? What was the ambient temp? Any wind? Did you try using a tuff test at the windows to see if the air flow was being sucked in/out of the car?

GRU · 07-10-2012, 03:28 PM

Temp was about 75 with very little wind. I did not do tuft testing because i was just trying to see how much do open windows affect fuel economy.

The test was caosting from 70km/h (about 45mph) so maybe i would see a difference around 60mph

Also, i know i should do a average MPG test with windows open/closed but the cruise control isn't working right now

Sven7 · 07-10-2012, 03:33 PM

45? Do at least 60, if not 70. The faster you go, the more aeromods show. And did you coast to a stop, or just down to 20mph or so?

Gasoline Fumes · 07-10-2012, 04:14 PM

I wasn't very scientific about it, but I tested this recently. There's a hill I can coast down, starting at 50 MPH and ending at a stop sign. I usually end up going around 22-23 MPH where the road levels off before I need to brake. Windows are usually closed or slightly open. I tried it with all 4 windows open in the Wagon. My speed dropped down to 20 MPH. All done on different days, but the windows closed speed of 22-23 MPH has been pretty consistent.

My friend's heavier and more aero 2002 Jetta only drops to 35 MPG on the same road.

mcrews · 07-10-2012, 04:17 PM

I ran my 4 door jag (99 Vaden Plas) with windows open and windows closed several yrs ago on a stretch of highway at 65mph (on cruise)10 miles up and 10 miles back , 4 trips. using the scangauge the window down cost me 3mpg on a car that did 26mpg on the road.