Comment 80 for bug 28052

It feels like "Acceleration" in Gnome control center corresponds to the constant multiplier "acceleration" in xset, and "Sensitivity" in Gnome corresponds to "threshold" in xset. If you set acceleration high and sensitivity low, it's impossible to move one pixel at a time. If you set sensitivity high, you can move within a large range one pixel at a time before it starts jumping. If you set acceleration to "slow" and sensitivity to high, it actually slows down if you move faster.

This two-step acceleration is pretty much garbage for a real mouse, but works somewhat for a touchpad. For a real mouse, I have found that "xset m 11/8 0" works best, using the hidden "set threshold to 0" feature that uses a smoothly-varying acceleration instead of the jerky two-step.

See http://ubuntuforums.org/showthread.php?t=748412#4