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Issues of bigger tire size is not so much about speedo (it's just often brought up), but rather about 1) rotational weight increase (even if overall tire+weight stays somewhat same, due weight being further from center it needs more energy to spin up/slow down, and also even while one can get quality 18" wheels that are lighter then stock 17", good 17 of same model will weight less anyway) & 2) lowered gearing ratio (less engine rpm-s per same distance traveled).
Given relatively low power if staying NA it's best if you keep that rotational unsprung weight down to not get slight decreases in acceleration/braking (wheels are like some 4 flywheels. You spend extra engine energy to speed up them, and extra brake to slow their rotation down). & suspension quality (the less tire+wheel weights, the less mass/inertia it has & the easier for same suspension to get it down after hop, resulting in more grip, less unsettled ride). These reasons are more important then speed increase at same speedo reading (which as other noted, by car manufacturers agreement for new cars always is read lower even at completely stock).
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