You can't do it like that. The 555 does not directly replace the hall counter. You must set it up as a timer to send pulses at the same rate that the hall sensor would have sent them at the wheel speed you want the timer to emulate (such as 40MPH). You'll have to calculate that out for your application. There are a lot of 555 tutorials on the web, with example circuits, and even web-based component calculators. Just google for "555 tutorial". Lots will come up, like the good one you found and linked to already.
The 555's power pin can hook up to where the hall sensor's pin used to, and the 555's ground pin can hook up to where the hall sensor's ground used to. The 555's output pin will hook up to where the hall sensor's output pin used to. All it's other pins go wherever your calculated components and hookups go, based on how you decide to set it up.
Keep in mind that you will need to make a very short pulse width for the 555's output, and if you can't make it short enough with just one 555, you can use a 556 that has two 555's in it and use one to generate the timing pulses in "astable mode", and the second in "monostable mode" to shape those pulses to the correct shortness. Again, the tutorials will show you how to do that, and help you calculate the values for the resistors and capacitors you'll need.
Keep in mind with any timer replacing the hall sensor is that you will not be able to correctly see the display except when driving at the exact speed the timer was designed for. Meaning, almost none of the time will you be able to see it correctly.
The only way I can imagine you could do this so that it will work as it was originally designed, without using some sort of rotational sensor like a hall or reed switch, etc, is to use an accellerometer, and rewrite the program to read it to determine the speed. There is another thread somewhere around here where someone was talking about doing just that, although it was for a free-spun by hand version, I think. Don't remember what the thread was called, so you'd have to look for it.