You need to disable vsync like this when running glxgears:
vblank_mode=0 glxgears
For Nvidia drivers do this:
__GL_SYNC_TO_VBLANK=0 glxgears
Notice the higher than 59-60 fps results with vblank_mode=0:
ATTENTION: default value of option vblank_mode overridden by environment.
7919 frames in 5.0 seconds = 1583.704 FPS
8187 frames in 5.0 seconds = 1637.266 FPS........