genuine on my vitz 2006 was 165/70/R14, which i replaced with 185/60/R15.
the ride got even more smoother and the car now looks better than before. sometimes on R14's my car would hit a speed-breaker when 2 ppl sitting in the back. Now never happend.
My car is still on petrol, so there is no extra weight of a CNG cylinder in the back..