This situation came up yesterday in a game. I wasn't sure what to do and checking the rules didn't yield a clear answer.

You move a ship, it overlaps another ship (rams). You drop your speed back until it no longer overlaps. In this case, it dropped all the way to speed zero. Now the speed dial is unchanged, I know that. But I shot at the ship that had done the ramming that turn. Is that ship at speed zero until the next turn, and therefore can't use any defense tokens? Or is it considered to be at whatever speed is on the dial?

It is temporarily changed to zero as in, for that instance you can only move so far without overlapping the other ship.

Basically it says, if it overlaps go as far as you can without hitting the other ship. The speed would remain the same. It is poorly worded.

The easiest way I can explain is is that your actual speed dial never changes. If that dial is 0 you don't get defense tokens.

Page 8 of the RRG:

temporarily reduce its speed by one (without changing the speed dial) and move the ship at the new speed. This process continues until the ship can finish its maneuver, even if that maneuver is to remain in place at speed “0.” Then deal one facedown damage card to the ship that moved and the closest ship that it overlapped.

Then the first bullet point:

If a ship temporarily reduces its speed, its speed returns to the number indicated on the speed dial after it finishes executing its maneuver.

So even if you had to go all the way down to Speed 0, once that is done, your speed returns to what it was and so on the next turn, you are NOT at Speed 0.

As others have said your speed returns after the maneuver to what it was originally, so when the next ship activates and fires at you you are at whatever speed is on the dial and can therefore use defence tokens.