Ah, PolitiFact. Where "we couldn't find any good data" is proof that a claim is false. Note, I am not suggesting that the claim is true, either--I also don't have any actual data to bring to the table. While I agree the burden should rest on the person making the claim, the actual status here is "undetermined" rather than "false."
I would also note that your 13% number, picked from that "fact check" is sources from data that is specifically marked as "unreliable" which is a pretty strange thing to do when "checking facts":
But the Center had concerns about the reliability of its gang-related homicide numbers and stopped collecting them after 2012. It put an asterisk on top of its chart with the note urging "caution" in interpreting the results. In footnotes, the Center explained that localities had different ways of defining the term "gang-related."
With all of the above said, I felt motivated to try to answer this question. Like Politifact, I, too, found that this data is simply not reported in any kind of standardized way. You can, however, find some if you look--some jurisdictions DO do this work on their own. Contemporary with the data in the PolitiFact article is the Chicago PD's 2011 murder report. The conclusion drawn from this (data on pages 27-28) is that a full third of murders in the city of Chicago were "gang related" (either "gang altercation" or "narcotics.")
There are no specific "murder reports" available post 2011. However, data does exist in their annual reports. The most recent of which, for 2021 states on page 85:
In 2021, of the 800 criminal homicides, with the known motives, 49% were reported as death from “Gang Altercation.”
While this is certainly not 90%, it's almost four times your claimed number. Further, that 49% number is a floor as of those 800 homicides, a full 35% of them have motive listed as "unknown."
While Chicago is certainly not a proxy for the entire US, and "the murder rate would drop to almost zero" if you "removed with an x-acto knife specific neighborhoods" is at the very least hyperbole, the data suggest that "gang related" is certainly a major contributing factor, to a far greater extent than you claim in an attempt to refute "lies and utter nonsense."
Man, I hate getting drawn into religious wars, but I'm a bit of a number nerd, so:
That Chicago PD report referenced said
800 murders
515 murders with known motives.
of the 515 murders with known motives, 253 or 49.1% were gang-related
So, the 49.1% isn't the floor, 253 is the floor, so by percentage 253/800*100 or 31.6% is the floor. It is a mistake to make any assumption about the makeup of murders where no motive is assigned, in the absence of other data.
So, it is not 13%, but it is nowhere near 90% either. Using a geometric mean (more applicable to ratios than an arithmetic average) of the two guesses we get 34.2%, so if you want to assign "wrong-ness" values to the guesses, 90% skews slightly more wrong (less right?) than 13%.
