Rear pulley

Hi,
I pulley swapped my mower and put a 3.25-inch pulley in the rear and a 6 inch on the motor. I got a little over 20 MPH out of it. This has 18-inch wheels now but I am putting 22.5-inch tires on in a bit. I want to put a smaller rear pulley but I am scared it will slip. Is there a way to put something grippy in the pulley to eliminate slip with a smaller pulley? I am thinking around a 2.5 inch pulley.

Not sure of anything to make it grip better other than a good clutch spring.  Couple things to consider with the smaller pulley.  

Make sure your front end is ready for the speed.  Death wobble is not your friend. Lock the front axle in place where it does not pivot, or have good suspension.  No wheel play at higher speeds.  Like death wobble, your tractor turning when you have the steering wheel straight or you turn and the wheels jerk.  Just looking for a high speed roll over.  Not saying this won't happen with fixed axle or suspension.

Also, smaller pulley on the transaxle will put more strain on the input shaft.  Be prepared for that to break or go out faster.

Just an FYI.