Up in northern BC, when my GPS was always telling me "300 miles to the next gas", you'd be amazed at how many gas stations you run across.
I do remember the 3 of us booking past a gas station, having only done 40 or 50 miles since our last fill-up, and then passing a blue sign saying "Next Gas 160km" ... I laughed to myself, as all 3 of us cut the throttle, all 3 heads dropped, looking at how many miles we had gone, trying to convert km to miles, and trying to figure out if we could make it that many (converted) miles.
Of course, after we filled up, it turned out there *was* a gas station a lot sooner than 160km.
The first rule of thumb is: if you have a doubt in your mind, fill up...
The other rule of thumb is: ... there is always gas where you don't expect it.