I'm changing my sprockets to give me more top end power (but really to lower my highway RPM's). In the process I did some calculations for top speed. These calculations consider running at 14,000RPM redline in 6th gear on a stock 2001 bike. Other years may have different transmission gearing. I know the early Ninja 250's had a different rear sprocket. These calculations do not consider aerodynamics, friction, weight, horsepower, or any of the very real factors that affect top speed. I'm only looking at "if you're doing 14,000 RPM's in 6th gear, how fast are you going?" Anyway, here's what I came up with:
The transmission has a primary reduction of 71 teeth to 23 teeth. Sixth gear has a ratio 25 teeth to 28 teeth. The front sprocket has 14 teeth. The rear sprocket has 45 teeth. So to determine the RPM's of the rear wheel, just multiply 14,000RPM's * 23/71 *28/25 * 14/45. That gives you 1,580RPM's. This part of the calculation is quite precise.
Now you just need to know the circumference of the wheel to determine speed. I calculated the theoretical circumference, not taking into account a) that tire radius is less at the bottom than anywhere else, b) that the radius increases at highway speed, c) that radius gets smaller with tire wear, or d) that radius increases with tire pressure (which also increases with temperature). I also assumed that the radius is exactly the rim radius plus the tire radius (which it is not). Anyway, here's what I got for radius:
The rim is 16 inches, so the radius of the rim is 8 inches. The tire is a 130/80/16, so its width is 130mm, and its height is 130*80/100mm, which is 104mm or 4.094 inches. So the combined radius of the rim and tire is 12.094 inches. The circumference is 2*pi*r, which is 75.99 inches.
So 1,580 rotations/minute * 75.99 inches/rotation = 120,087 inches per minute. Multiply that by 60minutes/hour to get inches per hour. Divide that by 12 inches/foot to get feet per hour. Divide that by 5,280 feet/mile to get miles per hour.
And the answer is . . . (drum roll, please) . . . 114mph.
Naturally, that is assuming that you have the aerodynamics, horsepower, straight safe road, etc. to actually make it to 14,000 in sixth gear. So what this calculation really means is not that you can make it to 114mph, but rather that you can't go over 114mph without changing the sprockets or rear tire size.
And just for fun, if you change your front sprocket to 17 teeth, and your rear sprocket to 33 teeth, and you put on a 140/90/16 tire, then you can't go faster than 202mph, which we all knew already.
Isn't math fun?
Curt