I pass most of the time no matter where I am. But I have noticed that I pass with a lot more certainty when I am in the US. I've been to Las Vegas (and other parts of Nevada), as well as Indiana. When I am there, no one thinks I am female. No one. They don't seem to pick up on the fact I am trans either as the "sirs" and whatnot come with no hesitation at all.
I also seemed to have no trouble passing in a (Canadian) city not so far from me, but I am not there often so I can't say.
But when I am here (I live in western Canada), I do pass, but they seem a little less certain. The "sirs" come with a bit more hesitation. Some people seem to avoid pronouns entirely, to the point where it comes off as a bit awkward. Some people make indirect references to gay or trans issues, and I can't help but wonder if it's as if to say "I can tell you're trans and I'm accepting."
So what gives? Is it just in my head? I find it really strange. I know some people know me from before or have seen me early in transition, and I can excuse that (I'm a fairly memorable customer as I have "one of those faces" and I'm generally extra friendly). But people who have never met me before seem less certain here...whereas strangers in other towns don't even seem to question my manhood.