I am going from a 11 tooth on the front to a 12 tooth how many mph will that gain. The rear is a 46 tooth.

 

Insufficient data to provide an answer......

 

We must know the rear sprocket also to calculate the delta of the ratio and find percentage of gain.

I reckon about a ten percent drop in RPM for any given speed. Change in top speed depends on if it will pull the new ratio to the same old max RPM.

 

Very simple to calculate.. your original was 11 tooth, your new is 12 tooth, so your speed will increase with a ratio of 12/11, which is 1.091, which means about a 9.1% increase in speed. That means if your on say a 400EX that stock will do say 65MPH (that's what I've heard), your new top speed will be 65mph*1.09 = 71mph. This is only, of course, if you are talkin about RPM limited top speed. You will at some point reach a limit where you simply don't have enough horsepower to push the quad any faster.