if you wanna measure, take a ruler and make sure it goes through the middle of the pulley. the 3.8" is the diameter unless i'm mistaken. and honestly, you're probably not getting the best performance with the 160 thermo and the colder plugs with a stock pulley. if you do in fact have a zzp tune, a 3.4 pulley should be fine. the best way to know is to put one on and scan for KR