Nice diagram! There are many possible solutions. Here's one:

Treat the two ropes going up at an angle as one rope and tie them to
the vertical rope above them with a bend like a Rosendahl Bend
(Zeppelin Bend):

http://www.geocities.com/roo_two/Zeppelin.html

Imagine the gray rope as the two ropes coming from the tire, and the
white rope as coming from the limb. It shouldn't take too much line
and shouldn't jam so that you can adjust the ropes to level things up
later if needed.
Cheers,
roo
--See the Notable Knot Index for my real e-mail address