I just bought a kfx 400 and its about due for tires i plan to do some stock class racing with it. I wanted to see what tire you suggest, and what size? also if theres any up grades u suggest please let me know thx!
It has dg nerf bars, dg front bumper, dg exhust, and its been jetted!

 

Well for the size just keep the stock size..... stick with 20x11x9 in the rear.... I have seen some people switch to 21" fronts, since stock is 22" but I'm not sure you want to do that because you loose some ground clearance in the front.

As far as what brand/type I think you will get a bunch of different opinions, depending on where people live.

I run Maxxis i-RAZR radials on my z400....Mostly for two reasons -

1) because I live in NY state and we get a lot of mud for a lot of the year. And the wide spacing of the rear lugs of the i-Razr really works well in the mud. Some people around here will run ITP mud lites all year around since there are some years where we get mud all season.

2) because I have found that a radial really soaks up the hits in the front end at high speeds and bounces off rocks with less feedback through the bars.

However - as much as I like radials on the front, I don't think they make much difference in the rear. And during a more dry season I would opt for the RAZR-2 in the rear, as several of my friends run them and they are monsters in the loam. IMO they slide better than any other tire out there.

There is also a new Bill Ballance tire by Maxxis that looks promising as an XC tire.

If you are going to race motocross, or ride in the desert, most people will tell you to look at the ITP Holeshot line. They have a huge variety of tires and all have great reputations.

 

I never ran any aftermarket sport tire other than Kenda Klaws that I had on my Raptor. They were a perfect tire for me. The rears have a v-shaped tread design that allows you to flip them around backwards for soft terrains. I even used them at Silver Lake sand dunes, and they did just fine. They hook up when you need it and slide pretty good too. Never had a flat other than when I hit a big rock with the front tire racing around in the woods and pinched the sidewall. One of my friends has Razr rears and they seem to loose traction a little easier than my Klaws did.