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I have a 2011 Toyota Prius, and where I live there are only two types of gasoline: 90 and 95 (RON).
In the refueling section in the car manual it says: "Use unleaded gasoline. (Octane Rating 87 [Research Octane Number 91] or higher)"
Does 90 RON correspond to a rating lower than 87?
Can I use the 90 RON gas? What are the consequences if I do so?
For a good explanation of RON vs MON for gasoline octane, see Refining Online.
Since your vehicle requires Octane Rating 87 (which is probably (RON+MON)/2) or RON 91, and your fuel choices are RON 90 and RON 95, then technically it is most likely that only RON 95 will meet the requirement.
Why? If the manufacturer considers RON 91 to be about Octane Rating 87, then RON 90 will likely be less than 87. There is no direct conversion possible, because these are measurements, and not absolute units.
Will it matter? Hard to know. Using a lower octane fuel than recommended can result in the fuel/air mixture self-igniting (commonly called "knocking").
In a non-turbo, non- high performance engine, there's a lot more leeway when it comes to low-octane fuel. However, the Prius has a pretty high compression ratio (13:1), so it will be more sensitive to low octane fuel than if it were around 11:1 or even 12:1. But as others have said, the knock sensor would prevent serious problems. Plus knock usually only happens when you're driving hard and putting the engine under lots of stress (ergo heat). If you fill up with low octane fuel, don't race around like an idiot and you should be fine.
