Closing in on 65 years old here, driving for 50 years. Bought my 2015 May 9, 2015. 3800 miles 54.4 mpg average since new and I have not changed a thing. Mine is the base DE USA model.
My advice is to use it as is and learn to drive instinctively. Learn situational awareness and anticipating danger. These things are crucial for your survival and to avoid trouble, even when it is totally the other persons fault. I have pulled off the road to give room to the driver behind me to keep them from hitting me in the rear end and they thanked me for my awareness of the danger and saving them (an me) a lot of heartache.
There is no rush to modify and well though out mods are better than those soon regretted.
regards
mech
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View my fuel log 2015 Mirage de 1.2 manual: 55.7 mpg (US) ... 23.7 km/L ... 4.2 L/100 km ... 66.9 mpg (Imp)