42/14 = 3 or 3 to 1. This is the stock gearing ratio. Meaning for every 3 full revolution the front makes the rear sprocket will make one full revolution.

42/13 = 3.230 or 3.230 to 1. Meaning for every 3.230 revolution the front makes the rear sprocket will make one full revolution. Or 3.230 divided by 3 (Stock Gear Ratio) = 1.076 or 7.6% gear reduction over stock. So if you are going 40 MPH at say 5000 RPM with stock gearing. With the 13 tooth front the engine will have to be running at 5380 RPM to go the same 40 MPH. 5000 RPM x 7.6% = 380 + 5000 = 5380 RPM.

42/14 = 3 (Stock)
45/14 = 3.21. 3.21 divided by 3 (stock) = 1.071 or 7.1% Gear Reduction.
46/14 = 3.285 = 1.095 or 9.5% Gear Reduction.
47/14 = 3.357 = 1.119 or 11.9% Gear Reduction.
48/14 = 3.428 = 1.142 or 14.2% Gear Reduction.
49/14 = 3.500 = 1.166 or 16.6% Gear Reduction.

42/13 = 3.230 = 1.076 or 7.6% Gear Reduction.
45/13 = 3.461 = 1.153 or 15.3% Gear Reduction.
46/13 = 3.538 = 1.179 or 17.9% Gear Reduction.
47/13 = 3.615 = 1.205 or 20.5% Gear Reduction.
48/13 = 3.692 = 1.230 or 23.0% Gear Reduction.
49/13 = 3.769 = 1.256 or 25.6% Gear Reduction.

 

That's pretty close to my KISS approach...

 

So really, even assuming a big fat 10% increase in rpms. At 7000 rpm cruising speed on the slab, the new revs will only be 7700. That's nothing when our idle to redline is a 9000 RPM spread.

 

Ill tell you where! I have an AFR guage AEM EUGO WideBand Installed on my KLX with stock jetting,stock box, and an HMF race slip on. WOT Im at 11-12 AFR, thats rich as ****! Half throttle im at 13 AFR solid, that still kinda rich. at cruise 1/4 throttle im at stoich 14 +/- idle at 13ish.

And i got the sprocket info from the dealer i bought it from. and the show room 09 klx s had 13t i counted and the sf had 14t sittin right next to it. I was gonna buy a 14t but he told me it already came with so i bought a 15t for the hwy.

 

Swo, you are referring to an '09 on the CS sprocket, and I won't claim any absolute knowledge on it vs. '06-'07. On the jetting, I've ridden a couple of bone stock '09 KLX250S', and they are extremely lean. I know you said you have a guage on yours to detect mixture, but that sounds a little odd...especially after installing what I assume is a less restrictive slip-on muffler. Swo, just about every street legal, carbed, DS bike for the last few years have all been extremely lean to meet emissions. But hey...this is probably something that internet jousting can't qualify anyway...LOL!

 

First thing I would suspect is that your gauge is wrong or inaccurate. Quite obvious that these bikes are lean from the factory.

Also, just becuase perfect combustion in a lab results in that 14:1 ratio does not mean that we want a 14:1 ratio at the exhaust. Is your smog equipment still installed?

 

Stock front sprocket on an 09 KLX250S is 14t.