"...As one moves up the V-scale, the indoor grades become a better
representation of difficulty encountered outside. For example, some
outdoor V6s with excellent friction might feel equal in difficulty to a
few of your indoor V6 projects. This is where the route’s style—crimps,
slopers, pinches, compression, etc.—comes into play. If you're a
crimping fiend, a line of V6 crimps outside may feel easier than the
overhung pinch-fest V6 that’s shut you down in the boulder cave for the
past month. And there will occasionally be the outdoor V8 that you send
in half the time that you struggled on a gym V7 in your anti-style. It is rare for gyms to set problems harder than V10 or 11, as most of their clientele can't climb the grade, though outdoor grades currently extend up to V17. If you aren’t currently flashing every problem in your local gym, your time is better spent training than worrying about the day you outclimb the facility.
Don’t let challenging outdoor problems take the fun out of climbing. It takes several sessions to adapt to the texture, holds, and movement of real rock, so go into your first few outdoor bouldering sessions with an open mind. Set a goal of having fun and enjoying nature instead of trying to send your hardest V-grade yet. Soon you might be climbing harder outside than in the gym, and it’s important to remember that there is much more to climbing than pulling plastic in a chalk-covered, pay-per-month facility. "
source:
http://www.climbing.com/skills/bouldering-grades-gym-versus-outside/
while you're at it peep this science too:
http://www.rockandice.com/how-to-climb/best-rock-climbing-ethics-and-practices
