There are two extremes...
In cave country, a 3600 psi fill in a 2400 psi 3AA steel tank is common - and that's when it cools, so the actual end pressure in the filling process is between 3800 and 4000 psi, depending on how fast they fill it and whether a date bath is used.
3AA steel tanks have a very generous safety factor, and they don't have the fatigue issues of aluminum tanks. As long as the tank does not exceed its elastic limits, it will return to its original shape and won't be harmed by an over fill. People will argue the point, but a couple decades of overfilling to nearly the test limit (4,000 psi) in N FL has yet to produce a catastrophic tank failure. ANd this is often on tanks that are, by my definition poorly maintained and often not visually inspected on an annual basis. About the only thing I've seen consistently enforced in FL is the hydro test requirement.
In most of the rest of the country, tank monkeys seem to recoil in horror at the thought of exceeding the service pressure stamped on the shoulder even if it has a current "+" rating, and most don't want to exceed the pressure even during a hot fill. This basically means that an AL 80 that holds 77 cu ft at 3000 psi will be filled to 3000 psi, but as it is warm at the end of the fill, by the time it cools to room temperature, it will be down around 2800 psi, and only hold about 72 cu ft of gas. Obviously the tank is not "full".
Their (flawed) reasoning is that it is is illegal to overfill a tank and that exceeding the service pressure at any point is over filling. That basically ignores the DOT definition of "full" which is that the tank does not exceed the service pressure when the tank and it's contents are at room temperature.
Obviously, if an AL 80 is full at room temperature (3,000 psi, 77 cu ft, at 70 degrees) and you stand it outside in the hot sun in mid August so that it heats up to 120 degrees, it will still have exactly the same volume of gas. It may be around 3100 psi - in excess of the service pressure, but it's still not overfilled as the DOT and the engineering standard used assumes that pressure in tanks will rise if they get warm.
Given that the test pressure of an AL 80 is 5,000 psi, and that the burst disc (when new) will not rupture until the pressure is 90% to 100% of the test pressure (4,500-5,000 psi), nothing bad is going to happen with the tank out in the sun, in your trunk, etc. Those kinds of things are expected to be encountered in normal use and the engineering standards account for that.
No one really advocates long term and excessive overfilling aluminum tanks due to the fatigue traits of aluminum, but overshooting the service pressure by 200-300 psi so that you get an honest 3,000 psi when it cools is not a problem and it's also entirely legal.
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Special permit tanks like the 3442 psi X7-100 are a different matter. Pretty much no one overfills them and about all you'll get in N FL is a 3600 psi fill, like you would in a 2400 psi tank. That's done for good reason as they are designed to a different engineering standard and are a lot less "over built". They are for the most part very similar in wall thickness to a LP tank, and they are tested to only 3/2 the service pressure so they have less of a safety factor.
Unfortunately, many local dive shops won't fill a 3442 psi tank all the way to the 3600 psi it needs on a hot fill to cool to the 3442 psi range. Part of that is again a reluctance to consider that the gas and tank will cool to room temperature and it's the pressure at room temperature that counts. But part of it is just fill systems designed to accommodate AL 80s that they are reluctant or unable to push to 3500 psi.
Still, an under filled X7-100 will hold more gas than a fully filled AL 80, and the fill pressure has to drop all the way to 2650 psi in the X7-100 before you fall below the 77 cu ft you get in an AL 80.
Even a bad fill in an X7-100 is a lot better than a good fill in an AL 80, so don't get too upset over the potential for a 3000 psi fill - it's still 87 cu ft.
