John Mayer’s Ernie Ball Electric Guitar Strings
On electric guitars John seems to alternate between 10s and 11s, depending on the guitar itself. For instance, in an interview with MusicRadar John stated that his Black Strat handles 11s well, while some other guitar just becomes too stiff to play:
But that one [The Black One] just has a little extra slack; like a little leeway. Some guitars you’ll put 0.011s on, and it’s like [makes abrupt noise] and you just can’t move around. [John Mayer interview, MusicRadar]
In another interview with GuitarWorld John stated that the main point to focus on when choosing strings is whether you’re able to bend them comfortably or not:
’ll tell you, the argument about string gauges is about the silliest thing a guitarist can engage in. Maybe you get a better tone off of bigger strings, but if you can’t bend up to the note, what’s tone anyway? But like Hendrix probably had .010s, so it’s whatever you can bend. [John Mayer: Battle Star, GuitarWorld]
Based on this, and a tweet from Ernie Ball [Ernie Ball, @ernieball], John likely uses the Regular Slinky set (.010 – .046) on some guitars, and the Power Slinky set (.011 – .048) on the others – which as said includes the Black Strat.