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So how do these transmission variations affect how fast your bike will go?
It's pretty simple, really. Multiply the primary gear reduction by the
appropriate main/countershaft gear ratio, then multiply that number by the
ratio of the number of teeth on the countershaft sprocket to the number of
teeth on the rear sprocket. Still with me? Good. Now take that number and
multiply it by the engine RPM's to get the RPM's at the wheel. Figure out the
distance (in feet) your wheel travels in one revolution, multiply that number
by the rear wheel RPM's to get distance traveled per minute, then multiply
that number by 60 to get the distance traveled in an hour. That number is
feet traveled per hour, so divide by 5,280 to get miles traveled per hour.

Simple.

But even simpler is using my handy spreadsheet to let the computer do most
of the work. All you have to come up with is the number of teeth on the
appropriate gears inside your transmission.

Want to try out the spreadsheet?

<click here>

Keep in mind that there are inefficiencies in turning engine RPM's into ground
speed, so don't expect your bike to go quite as fast as what the spreadsheet
calculates. But it provides a relative idea of what gear ratios and sprocket
combinations do for overall speed.





		Ever wondered how your transmission compares to other bikes? Gearheads like me spend sleepless nights pondering these things, and gearheads with a lot of time on their hands, well, let's just say Excel tables can get created. The table below summarizes transmission ratios from a sampling of KTM's from 1999-02 and my 2003 KX250. The comparison starts with the primary drive, which is the gear connected to the cranskshaft. The table below shows that most of the bikes are initially geared down to about 1/3 of crankshaft speed. From there, further "gearing down" takes place at the main shaft and countershaft. In the table below, the numerator in the Primary Reduction ratio is the number of teeth on the sprocket attached to the crankshaft. The denominator is the number of teeth on the clutch sprocket. In the individual gear ratios, the numerator of the ratio is the number of teeth on the sprocket attached to the main shaft (same shaft on which the clutch spins) which corresponds to what gear you're in. The denominator is the number of teeth on the countershaft (same shaft that spins the countershaft sprocket), again, corresponding to what gear you're in.