speedo gear

The rear tire outside diameter affects the speedo reading and you didn't include that info. If you have a speedo gear currently in the car and it's just inaccurate, you can do this:
Drive the car with a GPS at a steady speed, say 70 MPH. Note what speed the GPS is actually showing. Take your existing speedo gear out of the trans and either read the number molded into the gear, or count the teeth.
Example: If you were driving at an indicated 70 and the GPS says you were going 62 MPH, your speedo is reading 13% high. (70 divided by 62 = 1.13) Multiply your number of teeth by 1.13 to get the new gear required. So let's say your current gear is 32 teeth, multiply that by 1.13 and you get 36.16. So the new gear needs to be 36 teeth. (you need more teeth to slow the speedo down) I did this with my 68 and it is now dead nuts accurate. I bought the gear off e-bay.