3 Gallons is probably right. I don't think they would delegate 1/5th of the tank to reserve by making the light come on at 5 miles. Hell, that's roughly 100 miles (at 20mph/gal) left in the tank.

That said- the light comes on- I head for a gas station. NOTHING is as ugly as a G35 sitting up on blocks, striped down to the frame along side a highway because the goober driving it ran out of gas!