One tooth wont make much difference but you do need to replace that chain too. You should be able to go +3 and still use a stock length
With a worn out chain I am sure he could go +5
One tooth will make a difference, more acceleration and you will be able to feel it.
you should really swap the chain and do the front sprocket while your at it.
front sprocket (-1 = 15 tooth)
If I'm not mistaken, -1 on the front equates to +3 on the rear, or vise versa, +1 in front will equal -3 on the rear. I did a -1 (went to a 16t) on the front last sprocket and chain swap and deffinatley felt the difference, quicker acceleration and she wants to stand up a little more. However, keep in mind that by changing the front sprocket you will also be changing the way your speedometer reads. I now have close to a 10% (to the slow side) error since I have not gotten a speedohealer yet. I dont believe changing the rear sprocket will effect your speed sensor, but I may be wrong.
If I'm not mistaken, -1 on the front equates to +3 on the rear, or vise versa, +1 in front will equal -3 on the rear. I did a -1 (went to a 16t) on the front last sprocket and chain swap and deffinatley felt the difference, quicker acceleration and she wants to stand up a little more. However, keep in mind that by changing the front sprocket you will also be changing the way your speedometer reads. I now have close to a 10% (to the slow side) error since I have not gotten a speedohealer yet. I dont believe changing the rear sprocket will effect your speed sensor, but I may be wrong.