Disable Vertical Sync in the options menu.
Vertical Sync basically makes you not be able to get more fps than the default that your monitor is able to produce per second (60hz for most monitors). Thats why you are not seeing over that much. Honestly, though, aside from just seeing how high it can go, I would just leave vertical sync on because then you will encounter screen tearing, in which the comp tries to put up two frames on the screen at once (because there are soo many your monitor itself cant keep up).
Disable it, to see you go up to 100fps and then re enable it if you encounter screen tearing. You will never actually be able to see over 40fps anyways.