I forgot to mention that where you buy the gas makes a difference too. With my last car I always bought Sunoco, but the ethanol content is too high which reduces fuel mileage. I have found Shell to be the most efficient, Petro Canada & Esso are tied for 2nd, and of course Sunoco. I do not have any accurate instrumentation to meaure the performance change but didn't notice any differences in the cars performance.
I believe there is a thread in the 4 cylinder engine section regarding fuel mileage you can check for comparison
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