67 Barracuda Speedometer Questions

If you have a bad speedometer, take it apart. You will find plastic gears in the trio clock section. Point out to your customer that the 10ths wheel is in the trip clock. It's work load increases with every decade/digit/wheel it must turn. Thus the trip clock shortens the life of the instrument. Even without the trip clock, good 60 year old clocks are not common.
When sellers offer a used speedometer they place it on a table to photograph. The instruments weight was resting on the input. Take notice of how far this pressure lifts the needle. That is your evidence of how much wear is in the instrument. When it was new that small bias force would not lift the needle at all. If I see more than 5 mph lift I keep on shopping.
If your customer absolutely must have 150 mph plus trip, inform him that restoration of that instrument may cost 200+USD. Good luck