hey piggy...
i have a 415... and i just ran across this thread...
those number you see in the back of the manual consist of what they call the "final drive ratio".
gearing your motor properly will not only extend its life, but also affect its performance.
your 415 comes with a 102 tooth spur gear. im not sure what pinion gear you have. different combinations allow u to get different ratios. a final drive ratio of 6.5 will have more top end... but a final drive ratio of 8.5 (these are extreme examples) will give you more punch BUT less top end. so play around with the ratios by using different pinion gears.
be careful tho.. if you "over gear" a motor.. you will cause it to over heat and burn, and it'll be useless. you can tell if a motor is over geared by touching the motor after you run it. if its too hot to hold for a few seconds, you've over geared it. hope this helps.