15t

hi, I'm heavily using my '05 fz6 in rush traffic, sometimes cruising hi-ways. Will 15t causes more fuel consumption as 16t? around 4-7k rpm when riding in city, 110 km/s (~65mph) when riding on hiway. I'm so tired if there are any linguistic errors, forgive me.

I just ordered a JT Sprockets JTF580.15 15T Steel Front Sprocket. My question is do I need a new chain and new rear sprocket as well? Is the stock chain a 530 or 520? I think that the sprocket I ordered is designed for the stock chain, but I'm not sure. Any input is appreciated.

Will a 2007 15t sprocket fit on my 2008??? It doesn't say on the package and the website is down for construction.