Hi City and all,
There are several ways systems can have thier channel assignments, you'll have to do your homework on any specific one.
The rule of thumb where repeaters are involved is F1 is through the primary repeater, F2 is simplex car to car on the output. Subsequent repeaters follow as F3 is the secondary repeater and F4 the output, so on and so forth. Often there is at least one simplex frequency unrelated to the repeaters.
Using the thumb on the other hand it is the same F1 and the others being through repeaters in surrounding municipalities for interoperability when officers are operating in another jurisdiction. Again there may be one or more unrelated simplex frequencies. This plan was once used in Union County NJ with F6 being common simplex used by detectives and while rounding up escapees from a nearby state prison. It stands to reason they didn't want this stuff broadcast over a wide area.
One important thing to remember is quite often those low power car to car freqencies are unlicensed so you won't find them in the FCC database. It's a perfectly legal way of operating on "secret channels" in an older analog non encrypted system.
One thing I should mention is when more than one band is in use the channel assignments can get scattered all over the place. Just because the channel selector has numbers on it that doesn't say they correspond to the radio assignments or the dispatcher's console. That's a good reason to do your homework, I have heard the radio repair techies wondering which way is up and it can get funny at times.