I am a big rider and it has been suggested that a wider rear tire is a good modification for those over 200 pounds (I'm 225 lbs.). I use the bike almost exclusively for off-road. What size tire is recommended - what is the maximum the KLX250S can accomodate?

 

We've had some pretty good discussions about the bigger-is-better theory here recently, but I won't even go into it. The short story is that I wouldn't recommend anything over 120/100, even at your weight. It's just not necessary. In fact at your weight and a bigger tire, I'd contend the bike could be quite sluggish off road...or on road.

 

The bigger the tire the more power it robs, Dunlop 606 120 is the biggest I would go.

 

I'm 225, running TM's 80/100/21 and 100/100/18. I don't plan on goin any bigger unless I get the 606 then 120

 

if you are going to go big in the dirt add,, a rim lock. once you drop the pressure and get it to hook up you will be ripping valve stems off.
You shouldn't have a problem with a stock bike unless you like dumping the clutch at high r's though because in stock form, the bike is a dog.
Start modding it and you will need a rim lock.