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Humming, I'm going to have to defer to someone else. My jury is out because I don't own a Jazz/Fit so it could be more advantageous for FE to upgrade or it might not. Its more a car specific thing than a general rule. Sorry it will increase FE of the engine but it might be more costly. You might get more MPG but it might cost more.
The only thing I can recommend is try a test of 2-3 tanks of each and compare keeping in mind any changes that you make either to the car or the driving.
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