Premium gas has an octane rating of 91 or greater, intended to keep the gasoline from exploding before the spark plug ignites it, a situation that car owners know as engine knock. Your 2004 Volvo XC90's owner's manual recommends you use premium gas. It also answers the questions of whether you can use less expensive regular gas -- fuel with an octane rating of 87.
The owner's manual says that, while paying top dollar for premium gas is best, you can run the car on less-expensive regular gas as long as the octane rating is at least 87, the standard for regular gas. You can determine this by looking at the label on the gas station pump. The 2004 XC90 is also equipped with an anti-knock sensor. This sensor tells your engine's computer if the XC90's engine develops a vibration in any of its cylinders, warning the computer that the engine may start knocking. When the engine's computer receives this warning, it retards the engine's timing to prevent engine knock. This can affect power and efficiency.
Volvo says that while using regular gas will not affect engine reliability, you should use premium "for best performance." You should also use premium fuel -- 91 octane -- for "demanding driving conditions," such as driving in hot weather or at high altitude, or pulling a trailer.
