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I did this speal one time on another forum:
OK this got a little long...
First you have to figure your 'total gearing'. Just divide the front sprocket by the rear sprocket.
ie. .................................................. . And just for fun
14/45 --- 15/41 --- 15/42 --- 15/43 --- 14/49
.311 ---- .3659 --- .357 ----- .3488 --- .2857
Then start with something 'known' like the STOCK 14/45 gearing (which overall gearing is .311)
1st - 36.1
2nd - 52.57
3rd - 66.75
4th - 81.1
5th - 94.1
6th - 105.4
Then just find the difference between two sets of gearing and multiply each gear: and then multiply the known gearing (ie. the 14/45 max RPM speeds) by that number
------- 14/45 ---- 15/41 ---- 15/42 ---- 15/43 ---- 14/49
--------1.0 ------ 1.176 ---- 1.147 ----- 1.121 ---- .9186
1st ---- 36.1 ----- 42.45 ---- 41.4 ----- 40.5 ------ 33.2
2nd --- 52.57 ----- 61.8 ----- 60.3 ----- 58.9 ----- 48.3
3rd ---- 66.75 ---- 78.5 ----- 76.5 ------ 74.8 ----- 61.3
4th ---- 81.1 ----- 95.4 ------ 93 ------ 90.9 ----- 74.5
5th ---- 94.1 ----- 110.6 ---- 107.9 ---- 105.5 ----- 86.4
6th ---- 105.4 ---- 123.9 ---- 120.9 ---- 118.1 ----- 96.8
Then to figure what RPM you'd be spinning in say 6th gear, just figure out what that speed is compared to the top speed, so...
---------14/45 ---- 15/41 ---- 15/42 ---- 15/43 ---- 14/49
6th ----- 105.4 ---- 123.9 ---- 120.9 ---- 118.1 ---- 96.8
65 mph - 61.6% --- 52.4% ---- 53.7% --- 55% ------ 67%
Then multiply THAT percentage by the max RPM (13,000) to figure out the RPM you'd be spinning at that speed
--------- 14/45 ------ 15/41 ----- 15/42 ------ 15/43 ------ 14/49
65 mph -- 61.6% ----- 52.4% ----- 53.7% ----- 55% ------- 67%
--------- 8000 RPM -- 6810 RPM -- 6981 RPM -- 7150 RPM -- 8710 RPM
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