How to determine correct number teeth speedo gear

Using your speeds, 40 and 55, with the existing 3.08. You likely have a 4.33 ratio gear set in the trans. From memory, seems like you can go one of two ways; inline gear matching device to speed up the drive to the speedo, or match a gear to the trans. Likely will need a different holding piece for the trans gear, as there is a finite number of gear choices for each gear holder. Each tooth on the driven speedo gear changes the speed 5-7 percent, depending on actual gear chosen. That's why the old speedo's were not exact, +/- ~3%.