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Gearing Calculator for your MAX Speed and Max HP

20380 Views 18 Replies 9 Participants Last post by  snapper08
Heres What weve got. I went to Sprocket specialties which is a sponsor for this site and did some math. I found the KFX gear ratios off a dirt bike site. This is assuming the dirt bike has the same ratio and Im not 100 percent certain. After doing the math I must say this calculator is damn close.


In order to use it right, you need the right numbers and here they are.

Front Sprocket Teeth : 14
Rear Sprocket Teeth: 38
chain pitch: 520
Front to rear axle distance: doesnt matter but it had 24.8 already in it.

Advanced info:
Engine minimum RPM: 2000
Engine Max RPM: 10300
Max Power RPM: 8700 (I got all this info off the dyno graphs)
Wheel rim size: 9
Tire Width Inches: 10 (guessing)
Tire Depth Percent: 27 ( I guessed cause I was tired of calculating)
Primary Drive ratio: 2.727
1st Gear Ratio: 1.750
2nd: Gear Ratio: 1.412
3rd Gear ratio: 1.188
4th Gear Ratio: 1.0
5th Gear Ratio: .875

Enter all this info and you will get the top speed I got when I used the GPS. Now go and change the Front Sprocket teeth to 15 and you get 73 which is what I got today on my GPS. So I hope this is close.

Heres what I came up with using there calculator for those of you who dont want to bother.

Remember, the stock gearing ratios are 14 front and 38 rear. A moose racing 15 tooth front sprocket for a KX450F was used on some of the tests.

With a 14 tooth front and 38 rear its 69.8 MPH (proven by GPS)
With 15 tooth front and 38 rear its 73.4 MPH(proven by GPS)

With a 16 tooth front sprocket(which I do not recommend by any means) and 38 rear is 78MPH
With a 14 tooth front sprocket and 37 rear its 70MPH
with a 14 front and 36 rear its saying 72MPH
with a 14 front and 35 rear its saying 74MPH
with a 14 front and 34 rear its saying 77 MPH
With a 14 front sprocket and 33 rear its saying 79MPH
With a 14 front 32 rear its saying 81 MPH

Now the KFX has a link so I guess you can cut it down.
I suggest keeping the stock front 14 for clearance reasons and downsizing the rear one.
Other than that I still dont have anyone to make the sprocket but I measured the center to center on the holes for the sprocket bub the best I could. I got 3 and 9/16ths of an inch and 3 and 5/8 at the most. I wasnt using the best equipment but tomarrow i will.

Ill let you all know tomarrow where I get my new sprockets and how much they are.

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THANKS for the info
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