Sensitivitity is 1:1 (6/11) by default. I think what you're talking about is mouse acceleration. If you're using Xfce or KDE, you can disable it in the GUI.
But if you're using Gnome 3 or you're not using Desktop Environment at all, you can try disabling mouse acceleration by using xinput (you need to have xinput package installed), but it might not work on the newest version of X.org. First, you need to get your mouse name or id:
user@localhost~$ xinput --list
⎡ Virtual core pointer id=2 [master pointer (3)]
⎜ ↳ Virtual core XTEST pointer id=4 [slave pointer (2)]
⎜ ↳ YOUR MOUSE NAME HERE id=8 [slave pointer (2)]
then you disable acceleration (with second command you can adjust your "real" mouse sensitivity, "1" is the equivalent of 6/11):
xinput --set-prop "YOUR MOUSE NAME HERE" "Device Accel Profile" -1
xinput --set-prop "YOUR MOUSE NAME HERE" "Device Accel Constant Deceleration" 1.000000
If that doesn't work, you can try using xset command, but it also might not work:
xset m 0 0
Finally, you can disable mouse acceleration completely in the X.org config as described in this guide.
P.S. Actually, my "gaming" mouse had acceleration disabled by default, but my cheap office mouse didn't.
P.P.S. Also there are guides for setting up mouse sensitivity if you want it: