If you can get your speedo apart, then you should be able to get to the flat coiled spring that is attached to the indicator needle. One end connects to the needle post and the other end connects to a (usually) movable piece.
Just move the movable end till you get the right speedo mph reading. The mileage indicator will not be affected, of course.
If your speedo comes apart, but you don't feel confident working with it, take it to a speedo shop. In the Phoenix area there are a couple, including the famous Dick's Speedo-Tach.
