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It depends. It might be worth a test run on both. I gather you started hypermiling when you switched? if thats true you ought to get a baseline on both and then tabulate the extra cost of higher octane against the lower octane FE and cost.
I'm going to gamble FE losses are as someone else pointed out that it could be the tires or the AC as well. I highly doubt the higher octane is going to yield a greater savings for you. then. . .if you really want to be ecologically friendly lately premium gas is not selling as high in demand as regular and therefore as long as there is excess premium being created and not consumed thats not ecologically friendly.
That said I'm not going to pay for premium just because exotics and high compressioners stopped buying gas. Stick with low octane get a diagnostic unit for your car, check tires and cut down on AC as much as possible(lowering windows slightly is better than AC if it still keeps you comfortable(all the way down is still more efficient(on most cars) than the AC on full blast)). If that diagnostic unit tells you its encountering knock all over the place I'd switch though.
Is the car supposed to run premium? I kinda of doubt it. I am pretty sure its a not too distant cousin of our FiT, and they don't require premmy fuel. Yeah stateside Fits require unleaded as the only requirement(lowest grade of fuel available here)
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