For goodness sake, listen to StephenH. You are working with a *diameter* of 27" to get your figure of 72 GI. For the distance, you need to convert that to circumference by multiplying by Pi, ~3.14159...(Remember Pi? Remember where it comes from? Hint Hint)

Short version- your only real mistake was in not converting wheel-tire diameter to circumference by multiplying by Pi.

For your purposes 3.14 will be fine. Want more accuracy? Adding more digits to the right of the decimal point will help almost none at all. If your really want more accuracy- you need to determine the "static loaded rolling radius" of your drive wheel. Do this by inflating your tires to riding pressure, at riding temperature. Then sit on your bike, with whatever gear & accessories you normally have, with bike held upright on level hard floor, & have someone measure from the floor to the rear wheel's axis of rotation- ie, the center of the axle. Multiply that measurement x 2 x Pi & that's about as close as you can get to your true loaded rolling diameter. I'd be surprised if it's exactly 13.5"(27/2) from the floor to the axle centerline when loaded.