I currently have 20s on the back and might want to put 18s on there. any advantages and disadvantages of that? and how would the 18s effect the speed.

 

well I actually did the same thing but on my 400ex. I really noticed a difference on the track and on the trails. As far as the speed I couldn't tell. But if your tires now are really bald the the smaller tires will feel like the same size because of the tread.

 

smaller rear tires will lower top end speed but give you more torque on the bottom end

 

....and the ride will be rougher (less rubber to cushion things) and of course you will lose ground clearance.

 

i had 18in holeshot tires on my raptor 660 and they sucked i would just stick with the 20s if i were u plus the 18in tires ruined my skidplate from having like 4inches of ground clearance