[t vago]

I'm not sure you're going to want to use a monostable 555 as a frequency divider. It is true that you can build a sort of frequency divider out of a 555, but it's only good at actual frequency division for a narrow range of frequencies. You feed it a variable frequency signal, like out of a VSS, and you're going to get wacky results.

Suppose you make a 555 circuit to divide by 2 at some frequency F. Now, at input frequencies below F/2, the circuit will not divide at all, but will pass that frequency F. Between input frequencies of F/2 and F, the circuit will divide, but it will not divide reliably by 2 until the input frequency comes close to the design frequency F. At input frequencies above F, the circuit will tend to always pass an output frequency of F/2 regardless of the input frequency.

Now, if your instrument cluster does not depend on an accurate VSS signal, then the 555 circuit would be good enough. In that case, I'd recommend a starting point of R1 = 3.6K ohms, and C1 = 2.2 uF. That should get you a designed F such that you'll see division by 2 at about 55 MPH. That's assuming, of course, that your VSS does spit out 8200 pulses per mile.