Placement is the single most overlooked variable in how well a hanging car perfume performs, and getting it right takes about ten extra seconds.
The rearview mirror is the default for a reason. It is central, at eye and nose level for most drivers, and generally clear of direct airflow from the vents, which means the fragrance diffuses steadily rather than getting blasted around unevenly. For most cars, this remains the best all-around spot.
String length matters more than people expect. Too short, and the fragrance sits too close to the mirror itself, restricting how freely it can diffuse into the cabin. Too long, and it can swing into the windshield or driver's sightline, which is more of a distraction than a fragrance problem. A length of about 2 to 3 inches between the mirror and the fragrance card is the sweet spot most people land on, close enough to stay out of the way, far enough to diffuse properly.
Avoid direct vent airflow if you want a slower, more even release. Hanging directly in front of an AC or heat vent will make the scent feel stronger in short bursts when the fan is running, but it also accelerates how quickly the fragrance depletes overall, which can shorten your total weeks of scent even if it feels more intense in the moment.
For larger vehicles, SUVs and trucks with more interior volume, a single hanging perfume in the front will diffuse less evenly to the back seats. If back seat passengers are part of the experience you care about, a second unit in the rear, on a grab handle or rear vent area, solves that without needing to double the intensity up front.
Avoid direct sunlight on the fragrance card itself where possible. Heat and UV exposure over time can degrade fragrance oils faster than normal ambient diffusion, which is more of a factor for cars parked outside daily in hot climates than for garage-kept vehicles.
None of this is complicated, but it is the difference between a fragrance that performs exactly as intended for the full 2 to 4 weeks and one that either underperforms or burns through faster than it should.