Harry, did you check the speedo that way before the adapter? the adapter correction isn't deviating, the speedo is, a spinning magnet is what makes the needle move, not direct drive gears - your speedo is as accurate now as it was before
"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir "When we tug a single thing on an old truck, we find it falls off" ~ me Some TF series details & TF heater pics