SwitchResX – Your displays
That's What Your Mac Can Do
SwitchResX is a clever tool and looks deep into detail. This advanced settings panel will let you customize all the data and frequencies your own machine is capable of. There are many options to choose from, including showing all defined current resolutions. A most advanced feature allows you to create new resolutions or customize existing ones !
Of course, there's always the security drop back "Restore factory settings" – just in case ;-)