A one-way journey from Hilo to Kona will take about 3 hours. Right on red is legal unless otherwise marked.
Gas stations are fairly far apart, and in rural areas, they tend to close early in the day.
Fill up at the first station you encounter when you start to run low. Do not attempt to drive Saddle Road, which connects Hilo to Kailua-Kona through the middle of the island. The road is poorly maintained and hazardous. Most rental agencies will charge extra if you rent a car in Hilo and drop it off in Kona, or vice versa. Rent a car as far in advance as possible.