Stock(ish) speed on 18 inches?

[Punkmaster Flex]

So I just bought my KFX400, but it's on 18 inches rear tires and I find it needs a 6th speed on straight lines. I would like to change the sprockets so I get about the same speed as a stock Z. Right now it's geared on stock sprockets, should I take one, two or three tooths off the rear sprocket??

[cataway]

http://home.earthlink.net/~gellett/ltz400.htm

[yellowhonda400]

just boy 20in rear tires. i had my kfx with 18s and it sucked. gearing was way off no power. unless your gonna race keep the stock gears and just get 20in tires.