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650 inches per sec / 12 = 54 feet per second x 60 = 3250 surface feet per minute . That is fast!
Lets try a shorthand calc
Simply, if the motor shaft pulley is 3" and the buffing wheel shaft pulley is 6" then the speed is reduced by 1/2.
3" divided by 6" = 2...so divide the rpm by 2 to get the rpm of the buffing wheel drive shaft 3450 / 2 = 1725 rpm
in your case
1.5" / 2.5" = 0.6
0.6 ( or 60 %) X 3450 = 2070 rpm on the buffing wheel drive shaft
To calculate the surface feet per minute of the buffing wheel
6" x Pi ( 3.146) = 18.876 ", the circumference of the buffing wheel
18.876 x 2070 rpm ( which is the rate the buffing wheel is turning)
18.876 x 2070 = 39073 surface inches per minute divided by 12 inches per foot = 3256 sfpm ( surface feet per minute)
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So, if you want to reduce your speed by 1/2 get a pulley twice as big. If you want to reduce your speed to 1/4 then get a pulley 4 times bigger, etc.
Hope this helps,;)
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Thanks Noswad, and Randy, I remember discussing this at the meetup.