I can't claim to know the details of the i3, but I am familiar with Gates timing belts. GT2 is a generic belt type. Within that group you can have different tooth sizes or pitches (the distance from one tooth to the next) from 2mm or 3mm up to 14mm pitch belts. I would measure the tooth pitch of your belt against the one on the i3. The larger the tooth pitch, the larger the belt.