If you are close, take RPM and devide by current gear and then multiply that number by new gear.
8000 / 5.83 = 1372.21 x 6.0 = 8233.26
7000 / 5.83 = 1200.69 x 6.0 = 7204.14
6000 / 5.83 = 1029.16 x 6.0 = 6174.96
This is just a rough estimate as many factors come into play but it is a number to base a change on.
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