Awesome, have a good day.
With a 6hp Tecumseh you won't be able to achieve gearing that will ever spin out the tires or sling mud and achieve 25mph. I was attempting to help you out with the gear calculator knowing full well you have a Torque Converter giving you told me you have a Manco 485, if it was using a standard centrifugal clutch it would be a 475.
So basically, your 485/Red Fox II/Gold Class II, LXT, etc model kart will not do what you want it to unless you buy a larger motor or gear the kart to basically do 10mph on top speed.