the storm st from memory has a deep tread pattern, 2 or 3 mm deeper than the normal tyres. Undoubtedly this affects the rolling radius.
I have a garmin zumo on my bike, and the speedo reads near on the same as the zumo with a new tyre. When the tyre is nearly shot it reads 10% high (indicated 95 is shown as 95 on gps).
From my experience, doing 90 miles to/from the south coast each day, tyre wear is noticable when maintaining higher speeds (e.g. 95mph). Drop it to near legal speeds and the tyres last much longer (like 1k miles longer). This means a new tyre every 10 weeks instead of every 8