I haven't yet used the reserve on my V-max because I live a block from a gas station now, but on my last motorcycle on vs. reserve definately makes a difference. (I know this isn't how the mechanics work but humor me on the example). Think of it like drinking out of a cup with a straw. If you leave the straw an inch off the bottom you will eventually start sucking air (this is "on"). From this point you can push the straw down that extra inch ("res") to access the rest of the liquid but the initial sucking of air at "on" was a warning that you were almost out. If you push the straw all the way down to to bottom of the cup (like putting it on "res"), once you hit the bottom of the cup, there is nothing more. You had access to all of the liquid and you drained the whole thing. Now you're screwed because you're out of gas on a busy freeway.