**Gear Inches**
One of the three comprehensive systems for numbering the gear values for bicycle gears. It is the equivalent diameter of the drive wheel on a

high-wheel bicycle. When chain-drive "safety" bikes came in, the same system was used, multiplying the drive wheel diameter by the sprocket ratio. It is very easy to calculate: the diameter of the drive wheel, times the size of the front sprocket divided by the size of the rear sprocket. (From Sheldon Brown's site)

The key being here that it is the diameter, not the circumference of the wheel. So you need a Pi factor in there somewhere.