My GN used to have one of those 145 mph stickers over the stock speedo face. To make the dial read correctly there was a gear reduction thing where the cable goes into the trans. This screwed up what the ECM sees as the true mph and I had problems with it shifting into OD. Got rid of all that stuff (the face sticker and the gear) and now all is well. My speedo checks exactly with my direct scan. Don't know if yours is anything like this but thats my $.02.