Slantnose speedometer question

[KipinMN]

Hi Everyone, until recently my 88 slantnose convertible showed about 60k miles on it and I noticed a few months ago that the speedometer works but is about 20 mph off meaning if it says I am going about 100 I am really doing about 70 mph. Then I wasn't too thrilled to see that the odometer went from a little over 60k miles to 660,000 miles. I am sure that is not going to help the resale but considering I have had it since new I am not too concerned about that. I have been researching threads and see I should be looking at the speedometer or the electrical sending unit but from what I have been able to provide I wanted to see which direction the members would suggest starting at first? I thought the electric sending unit but then I dont think that would have anything to do with the odometer but as I do not work too much on the car myself I really thought i would ask others first.

Thanks, Kip