Went with the "standard" gearing upgrade when I first got my bike. That was replacing the stock 14 tooth counter sprocket with a 13 tooth Sprocket Specialists P/N 514-XK13

This changed the stock gearing ratio from 3.0 (42/14) to 3.23 (42/13), it also allowed for retaining the stock chain.

After beginning to ride some off road trails, I concluded that I could benefit from an even lower gearing ratio - especially on the steep rocky downhills.

I did some online research and came across a website for Ironman Sprockets: http://www.dirttricks.com/ordering_sprockets.htm

Turns out that the rear sprockets on many of the Kawasaki dirt bikes and the KLX250S are interchangeable. Also in the short time that I have had a dirt bike, I have found that I'm brutal to them, so getting the most durable parts is important to me.

I got the 49T rear sprocket (P/N KX49) and a Regina ZRH Z-Ring chain (P/N 520ZRH/120) along with a new set of Grade 10 Sprocket Bolts and Nuts

These products are also marketed by MSR under their Hard Parts brand and I purchased these products locally at TriCity Cycle
Install was straightforward and the rear axle doesn't even need to be removed, you can sneak the rear sprockets past after loosening the axle nut, pulling the swingarm pins and pulling the wheel back (be careful of the rear brake line though).

The chain comes in a 120 link length and it needs to be cut down to 110 links for the 49/14 (3.5 ratio) setup. Counting the master link, you'll have 110 links, so the chain itself will have 109 links. The 3.5 ratio will also bring down the top speed. I usually trailer my bike to the trails, but also use the bike as a daily commuter - typically never faster than 50mph.

The rear chain guard needs removed to fit the 49T sprocket, but I removed that a long time ago. (Going to a 50T would require a small modification to the rear chain guide, so I'm told).
I cut the chain down with a cutting wheel on the dremel tool by cutting off the ends of the pins and then pressing the pin out with this great little tool that I got from my Dad many years ago:



I also used this tool to press on the master link so that I could get the clip to go on easily.

The only other issue that I had was with removal of the counter sprocket - but that's an entirely different story.

Here's the final results -- can't wait to get out on the trails to try it out ...maybe tomorrow!