It might not be 30 to 35 mph, but when you are riding it is going to seem like that is the top end. The 300 has plenty of low end to begin with. With a 12t you could be starting off in 2nd or 3rd gear with no problems.

 

Going from a 13 tooth sprocket to a 12 tooth drive sprocket will theoretically drop your top speed by exactly 9%. So if your 300 does 55 MPH now after the sprocket change it would go 50.05 MPH.

Divide the large sprocket by the smaller sprocket to find out the final gear ratio. For example 38 divided by 13 = 2.92 and 38 divided by 12 = 3.17

 

so i guess ill send the 12 tooth back. i have fmf exaust, k&n filter, dynojet kit 10.25/1 wisco piston and a cam. What size would be the best,stay stock, or go up a size istead of down