So your pre-boil gravity was 7.5 gallons at 1.056, this would give you 420 points of gravity total, which would put you at 1.084 OG if you had 5 gallons after your boil. Looks like you post-boil volume was 6.4 gallons or so to get you an OG of 1.066. It looks like your efficiency was fine, but you ended up with too much water/not enough boil off. Either that, or your gravity and/or volume measurements are off somewhere.
For future brews; you lost about 1.1 gallons of water from your pre-boil to your post boil volume, so that was your boil off amount, and you had a grain absorption of about 0.0625 gallons per pound of grain. Next time you brew, keep everything the same, but calculate your strike volume by:
target volume (5 gallons) + boil off (1.1 gallons) + grain absorption (0.0625 x grain bill in pounds) + losses from trub, etc. = strike water volume
Obviously, if you want to actually have 5.5 gallons in the fermentor, you would put that number in instead of 5 gallons.
Good luck!