[piloto]

2011.75 6MT, diameter as measured at 12 o'clock on the wheel is 1.55" (39.4 mm).

Measured again using vernier calipers (sorry, don't have access to a micrometer at the moment), and at the same location, I got a measurement of 38.4 mm.

The wheel is padded quite a bit with foam, unlike most other wheels. You can feel it compress. Controlling dimensions with foam is difficult. The leather wrapped around the wheel is stretched over the foam and then sewn. Depending on the patter of leather cut, and how tightly the leather needs to be pulled to get it to sit right, it could compress the foam more in some cases, which will result in a slightly thinner wheel. That would explain a 1 to 1.5mm variance. Something beyond that is likely attributed to a different factor.