[vyquad]

Going down to use a heidenau k66, Current plan is a 31t rear because i know that smaller wheel = more torque but less top speed but i need top speed since i go on highways with this bike

 

[redeemer1972]

Just leave it as is, you'll be fine.

 

[Emil]

According to Tire Size Comparison You are loosing 37 revs/mile on the new tire size compared to the original, (IIRC original sizing).
Then going to Sprocket Calculator - The easy motorcycle sprocket and chain calculator You might be able to make an informed decision.
I have 1st generation Rebels, so I always use original tire sizes but go with a 15 front & 30 rear configuration.
Do what works, but more importantly, what is safe!