Triumph Daytona 955i speedometer reading approximately 50% faster than it should

The previous owner said the clock (digital) was reading faster. It came with a Sprint RS speedo so I changed it for a 955i vairiant but still the same! The bike has a single sided swing arm with speedo sensor mounted on which is activated by the 6 nuts/bolts on the rear sprocket. On closer investigation I've noted some models although appearing to have the same speedo have conventional swing arm with 5 bolt sprockets and some have front wheel sensors! Could this just be a matter of pairing a speedo of the same year as my bike?? Thoughts and opinions welcome! Reg Neil