Have you ever felt like the middle mouse wheel zoom is too fast or too slow in AutoCAD? It can depend on the scale of the drawing and objects you are working on, or perhaps even your mood. This speed of the zoom in and out is controlled by a system variable named ZOOMFACTOR. The default of the system variable ZOOMFACTOR is 60. You type ZOOMFACTOR at the AutoCAD command line and then changing the value. My preference is 40 so that I am not zooming in too fast of increments.
This is another one of those little settings that allow you to make AutoCAD more tailored to your use whether you like warp speed zooms or turtle slow zooming with the mouse wheel.
You can also set someone else's ZOOMFACTOR to 3 just for fun or torment but be sure after watching them struggle to set it back. More Office AutoCAD Pranks
Another related system variable is ZOOMWHEEL which can change the direction of the mouse wheel zoom to match another product such as Autodesk Inventor. Change the Direction of the Middle Mouse Wheel Zoom in AutoCAD
Online AutoCAD 2010 documentation: