How to properly calculate wheel diameter (rim+tire)?
You could calculate the effective diameter by measuring rollout and dividing that number by pi.  To measure the rollout start with your valve stem in the lowest possible position and make a mark on the floor next to the valve stem.  Ride the bike one revolution until the valve stem is at the bottom of it's rotation again and make another mark on the floor.  Measure the distance between the two marks to find the effective circumference.