tire size help

i got a luna tire on my uni but i want a smother one for more speed and so i was looking at a 2.2 inch bmx freestyle tire and i was wondering if that would fit because my luna says its a 20 inch

no it wont… your luna tire is actually 19" and a bmx tire will be 20"

Just put more pressure in it.

Add more air to your Luna so you have a smaller “footprint”, a BMX tire will not fit a trials rim.