FIAT DUCATO 2.0LTR SPEEDO PROBLEM

gray0506

I RECENTLY BOUGHT THE ABOVE VEHICLE AND WAS TOLD THE SPEEDO WAS FAULTY ...IT HAS HAD A NEW/RECON BOX..I HAVE FITTED NEW SPEED SENSOR WHICH HAS CABLE DRIVE AND ELECTRIC SOCKET ?? ANYONE SHED LIGHT ON WHY IT STILL TELLS ME IM TRAVELLING FASTER THEA ACTUAL ..IE 30MPH 30MPH...40MPH /60MPH AND SO ON ...ALSO TRIP COUNTER AFFECTED ??