[dsy1]

Changing the speedo gear at the transmission is easy but it will affect both the speedometer and the odometer. I don't see how the speedo can be pretty close (although 4 MPH at 40 is still 10% off) but the miles read less than half of what they should. If the wheels or in the odometer have wear it may be that they are not advancing the way they should.

Does your speedo needle hold steady, or does it bounce some when you are at a steady speed? I had a bad odometer and the gears on the number wheels would bind up and not turn correctly, but that made the speedo needle bounce whenever the odometer wheels would stick.