“3” at the end of lines two and three represent the number of lines to scroll (“3” should be default) – modify this number to suit your needs. Once you’re done, save the file.
The first (“.*”) line from the code used above represents is used to specify in which applications to use those settings. “.*” means everywhere, but you can also apply application-specific settings by changing “.*” to the window name (and add multiple application-dependent settings). Run “man imwheel” for more information.
And finally, the last 4 lines in the code above are there to allow Ctrl / Shift with mouse scroll wheel up / down to work (for instance, to allow zooming in on a webpage in the web browser, etc.), which is the default behaviour.
3. Start “imwheel” (type “imwheel” in a terminal). Important note: make sure you don’t run multiple imwheel instances (if you want to stop any previous instances, run the following command: “killall imwheel”)!
4. Since these settings are only applied when imwheel is running, you should add imwheel to startup (if you use Unity, to do this, open Startup Applications, click “Add” and under both “Name” and “Command” fields, add “imwheel”).
It’s important to mention that imwheel applies the settings available in ~/.imwheelrc when the application starts and it doesn’t update when changing the configuration file. So if you modify the configuration file, you’ll need to close imwheel and start it again to apply / test the changes (you can close/kill it using the following command: “killall imwheel”).
this will most probably not fix issues such as very fast scrolling, if you encounter such an issue, you can try unplugging and then plugging the mouse back;
if you only want to change the scroll wheel speed in Firefox and Chrome, you can do this from within the app or using an extension: Firefox | Chrome, etc.
via AskUbuntu; http://www.webupd8.org/2015/12/how-to-change-mouse-scroll-wheel-speed.html