Your manual says 15 gallons. You fill up with the light on and the pump stops at 12.8. Nothing is wrong with the car.
The warning light comes on early
Automakers set the low-fuel light to come on with a reserve still in the tank, so you have time to find a station. How much is left differs from car to car; some manuals say, many don't. Either way, a fill-up from the light is the rated capacity minus that reserve.
The nozzle clicks off before the neck is full
The pump shuts off when fuel reaches a small sensing hole near the tip of the nozzle. That leaves the filler neck and some vapor space at the top of the tank empty, on purpose. Fuel expands as it warms up, and the evaporative emissions system needs that room.
Skip the extra squeeze
Topping off after the click can push liquid fuel into the charcoal canister that's meant to catch vapor. Repeated often enough, it can damage the canister and trigger a check-engine light. Stop at the first click.
So treat the rated capacity as a spec for comparing cars and estimating range, and expect real fill-ups to land a little under it.
