Don't use a measured radius to determine diameter in a case like this. Instead, use a measured circumference. Regardless of pressure, every time the tire rotates, all of the circumference has to come in contact with the ground.
Tire manufacturers often specify revolutions per mile at a specific speed in their spec charts. This is based on circumference, not diameter. If your tires are a readily available popular size, that could be of help. I just looked at BF Goodrich's website and they give the revs per mile at 45 mph. If your size isn't listed, try doing a "roll out" measurement. Mark the tire and move the car until the tire has made about 10 revolutions and divide the distance by number of revs to get an average circumference. Divide by pi to get the effective diameter.
