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.
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.
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
Want to try out the spreadsheet?
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.