"Incredibly unclear"? Really? Not the OP, but let's spell it out:
Manicuring a lawn is expensive and serves no useful purpose, so most practitioners have disposable income and/or spare time, and care about keeping up appearances.
Putting a car up on blocks implies it's being used for spare parts, which is generally not something the well-off need to engage in (they buy new and use professionals for repairs as needed), and having one on your front lawn in particular implies that you value spare parts more than appearances.
So both are reasonable proxies of wealth, and wealth correlates inversely with the kind of violent crime you could detect with gunshots.
Manicuring a lawn is expensive and serves no useful purpose, so most practitioners have disposable income and/or spare time, and care about keeping up appearances.
Putting a car up on blocks implies it's being used for spare parts, which is generally not something the well-off need to engage in (they buy new and use professionals for repairs as needed), and having one on your front lawn in particular implies that you value spare parts more than appearances.
So both are reasonable proxies of wealth, and wealth correlates inversely with the kind of violent crime you could detect with gunshots.