why not just use pi and figure out the exact circumference of the stock pulley and the bigger pulleys and at different RPMS... distance over time= speed... I am sure if I had the right tools to measure the exact diameter of the stock pulley and the S/C pulley there wouldn't be no guessing... math + physics = exact science... no opinions no other answers!... but I forgot schools now teach how to have high self esteem and integrate... not how to calculate the circumference of a circle or know what speed is

with the data YOU gave me(155mm crank, 74mm S/C pulley) this is what is correct for STOCK pulleys... in reallity with our cars... not ratios out of diameters difference...

the circumference is the distance the belt is moved after one rotaion.
the formula is 2xpixR... we already have 2R = 155(stock)
155pi = 3.141x155 = 485.855mm
74pi = 3.141x74 = 232.434mm
485.855mm/232.434mm = 2.090
so every rotation per minute = 2.090 rotations of the S/C

5800rpm(stock)= 5800x2.090 = 12122
6175rpm(tuned ecu)= 6175x2.090 = 12905.75

you can use these formulas to know what is really going on under there
I appreciate everybody trying to calculate and get a good answer why our max is so low and our stock is so high well I don't know the max but I KNOW the stock(if of course those pulley diameters you posted are correct)