Hi ! I'm from VietNam, sorry i'm not good at English much

My bike is Yamaha Exciter 2011 (Sniper - Jupiter Mx 2011), all stock !!!
I have two type of wheel:
---The first one, i use a standard wheel (front: 17 x 1.6, rear: 17 x
2.5), and standard tire (front: 70/90, rear: 100/70) and of course standard sprocket 14-38.
---The second one, i use a Yamaha ZR125's wheel (front: 17 x 1.6, rear: 17 x
1.85), and standard tire (front: 70/90, rear: 100/70) and of course standard sprocket 14-38. The ZR125's is much heavy than the standard wheel. Tire Circum-ference is little bigger than the standard wheel.
My question is, should i use sprocket 14-39 for the second tybe of wheel (ZR125) ?, does it decrease my top speed ?