Biggest likely cause are factors you can't control, traffic, weather etc... The other likely cause is how heavy your foot is. It isn't hard to get 30mpg especially if you idle a lot.
Doubtful, but you don't have a flat tire do you? It can be hard to tell if they're low on these cars sometimes, especially the rears.
Otherwise, what do you mean "in tune"? These things shake like crazy from new. They aren't balanced well. It's a 3 cylinder thing, the shake. If you had a miss you'd know it, it'd be noticeably low on power.
Did you notice your rpm being higher at certain speeds than before? Rough average for mine is around 2000rpm at 65ish mph, 1600rpm around 50mph. That's on flat road no wind, no ac on etc....light throttle.
Lastly, do you have oil in it?
Edit: ignore half of what I wrote, I just noticed you've owned your car from new and should know they shake by now.
Mirage videos:
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View my fuel log 2014 Mirage SE wussie cvt edition. 1.2 automatic: 37.7 mpg (US) ... 16.0 km/L ... 6.2 L/100 km ... 45.3 mpg (Imp)