sideways500

Hi there every one , i have a gs300 on a p reg 96 , it drives fine with smooth gear changes as it should but the over drive light is flashing constantley when it is on, and it doesent go in to overdrive until i give it some 6000 rpm , its as if it being forced in, ive had the oil check and the local garage said its fine, they pluged it in to there snap on devise and its comming up as a speed sensor ??? the speedo and every thing else works fine, can any one advise me regards, lee