I'm a newbie in sports quad? Do smaller rear tires give better acceleration? Thanks in advance.

 

Yes. A smaller diameter rear tire will LOWER your overall gearing, giving you better acceleration at the expense of some top end speed. You also lose some ground clearance in the process. If the atv has a chain drive, you can get the same effect by just swapping sprocket sizes(smaller on the front or larger in the rear) which allows you to keep ground clearance. This is also usually a cheaper option.

 

i have a kfx400. what rear sprocket size would you recommend. besides, is lowering your clearance by 1" significant if you mainly are on the mx tracks?

 

If you mainly ride mx, then going with the 18" rears is a better solution. Ground clearance shouldn't be a problem there and you get better handling. Plus the smaller mx style tires are lighter which always helps on the track. My brother runs 18's on his quad on the track and trail and is usually fine as long as the trails aren't real rocky or rutted.

 

my next question is: my kfx 400 rear tires use a 10" wide tire; would increasing the width to 11" make the ride more stable and is it advisable? thanks

 

11" wide is fine. You are likely getting more contact patch and so on. Most mx tires for the adult quads are 11" anyway, so you won't have much choice there.