I think Steve (Friscuba) makes a good point here. I get a lot of divers that tell me they will run out of air quickly for numerous reasons. However, when I guide them, they typically come back and say they just went on the longrest dive of their life. There are a couple reasons that this happens.
One: most of my dives are relatively shallow, less than 40 feet.
Two: I go pretty slow and keep my guests moving at a rate that keeps their need for air low, much like that of on land.
Three: I am able to watch for and prevent Free Flows. (How many new divers allow free flowing regs to go off for ten to fifteen seconds before they realize what is going on and finally stop it effectively? Got any idea how much bottom time you can lose from a free flow? A lot!!!)
I regularly gear up my divers in 63's, 72's and 80's. Once in a while I bring in a 100 at special request. Sometimes I use 50's (for the extra small divers). I don't think I (or my business) should be flamed for that. Do you? I have been at this game for 25 years now and for my dives, sizing up the tank to the size of the diver makes good sense. I personally use a 63 most of the time and always have the most air at the end. I take out many mixed groups (men, women and children on the same dive) and I've found that the kids and women in 63's, smaller men or teens in 72's and average to large men in 80's creates a situation where all stay near the same tank pressure throughout the dive and no one runs out particularly early and (usually) no one comes back with over 700-800 psi. This is also most ergonomically comfortable for the persons frame when it comes to walking around with the tank on.
To all you "Hoovers" out there, come to Maui and do a nice leisurely shore dive with me. I can almost guarantee I can show you a few things that will instantly improve your air consumption.
To comment on some of these prior posts:
I agree that runners and bikers typically use air way faster than other people. It's in the nature of their movement underwater that sucks it up so quickly. They're in shape so they buzz around quicker = more air use.
If an 80 gives you head wounds, you are simply mounting your BCD too low. The size of the crown of the tank/valve area (height of valve) is pretty much the same on all tanks that I use so it doesn't make sense to me that a 63 wouldn't hit you in the head but an 80 does. Do you know what I mean? When I use 80's I do sometimes notice the bottom hitting the back of my legs but I'd rather have that then have it hit me in the head with the reg./valve assembly.
I'm a big fan of 72's on boats, myself. Having almost as much air as an 80 but being noticeably lighter to load and carry, I am comfortable to use them. I do see where 80's are preferred though for the average recreational diver who doesn't breathe as efficiently as I do. Like it has been said here, call ahead to find out what the boat has and then you can decide without being dissapointed after it's too late. And though in a perfect world it should always be 3,000+ psi to start your dive, sometimes you get a fill that only has 2,850 or so. I think a good crew would be able to work around that and maybe give a 'low fill' tank to one of the DM's so to keep the fullest tanks for the customers. That's what I have always done when that happens. It shouldn't mean that the company should be given "maximum possible negative publicity" for a slight underfill or minimally leaking tank neck o-ring!!
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(I wonder if my wife will buy that?)!
