I performed the exact same procedure here- was reading about 4mph on the high side of actual and now is dang close according to GPS. Couple things to be aware of- having steady hands and a good set of dental picks (preferably a 90* angle one) will make this much easier than with big tools. Just pry the outer cap off one side at a time, then carefully work on the indicator needle. It'll kinda let go with a snap/bang, but it will come off. See below for detail of how the needle and cap assembly fit onto the speedo unit.