I had a yellow 250. I think the stock gearing was 15t front, seemed ridiculous. The back, being dished, was about unobtainable. (I think bob ginder had a kit with spacer to convert to flat rear sprockets.)
anyway I ended up with 11 or 12 front I think. I bought a 10T but remember that it was too small for the chain to clear the transm shaft or swing arm or something.
I digress, here's why I wrote: Get the transmission ratios from the manual, then divide the two ratio numbers of two gears, say first and second. That gives you the % spread between those gears.
I am picking numbers to use in this example (have no idea), say 15.00:1 in one gear, and 12.50:1 reduction in the next gear. Dividing these two is 15.00/12.50 = 1.20. This means they are 20% apart from each other.
Now, decide which gear you like. First is way too low, second is too fast, but I think second is closer to what I want. Therefore I want about 15% faster than low gear. Now you have some idea of the goal.
Just do the math from there.
If the original is 11/53, that is 4.818 reduction
12/53 would be 4.417 reduction, slightly faster.
Comparing the two is 4.817/4.417 = 1.09 or 9% increase in speed.
13/53 would be 4.078 reduction. 4.818/4.078 = 18% increase.
For decreasing speed, the math works the same. You are just looking at how much does it change compared to what I have now.
Maybe thats close enough. I would buy a couple front sprockets and try them. After that, if you want to change back ones you will likely only have to buy one, and it should be close.
k
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