I did a test with my drivers window up/down/up on three consecutive drag runs.
I posted these trap speeds:
1. window up - 108.14 MPH
2. window down - 107.68 MPH
3. window up - 108.69 MPH

All three runs were between 13.8-14 seconds

So the drag created by the windows does play a measureable role but I would only say by maybe 0-1.5 mph or so depending on if all the windows were down versus one or two.

These times of mine will be topped when I get my new engine project back together of course!

Former owner of '99 CSVT - Silver #222/2760 356/334 wHP/TQ at 10psi on pump gas! See My Mods '05 Volvo S40 Turbo 5 AWD with 6spd, Passion Red '06 Mazda5 Touring, 5spd,MTX, Black