you will change your gear ratio! one tooth LOL I honestly don't recommend changing JUST one sprocket or NOT replacing the chain at the same time, as they are wear items and them MATE with each other, when you change any of them and NOT all at same time, you loose that mating factor, and cause extra wear on all remaining parts then
Thus why its always best to change all 3 at same time
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Smaller front and bigger rear f will give you more low end grunt and less top speed
the smaller front will cause motor to rev faster due to it will turn faster
Bigger front and smaller rear will give you more top speed and less low end and slower rev's on gear/engine braking!
SO going smaller on REAR and leaving the front the same will change top speed a LITTLE
again, I recommend when you do a gear /sprocket change, , its best to change all 3 things at once, new chain and front and rear sprockets , NOT doing so will cause excessive wear and tear on all 3 parts, , accelerating wear
SO< unless your racing and changing sprockets to track conditions and HAVE money to burn
I suggest changing sprocket sizes when your due for a new chain and sprockets and change all 3 at that time
That's my 2 cents here, 35+ yrs of riding MX bikes with chains, and 30 yrs on street super sport bikes with some High HP
also recommend buying GOOD brand name chains and sprockets, the savings you get on cheap one, and the faster wear doesn't ever really add up to any savings!
secret to long life of a chain and sprockets is keep them clean and lubed and adjusted RIGHT, be it need to clean every ride, or adjust every ride(a TOP shelf chain won't stretch nearly as fast or wear as chap one's so adjustments are fewer)
just keep them clean, well lubed, and clean the right way too! and you will get a LOT of miles out of em!
As a general rule, changing one tooth on the front sprocket is roughly equal to changing the rear by four teeth!