Correct Speedometer for 73 Valiant

townsend said

President Nixon signed the maximum speed limit of 55mph in 1974 because of the gasoline crisis.For this reason Chrysler speedometers only went to 100mph starting in 1975. Both speedometers will give the same reading from 0mph to 100mph. Have you installed and tested the 100mph speedometer ?

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100% correct.
Also, I may add that the speedometer does not have anything to do with how the speed is presented on the dial.
The speedometer is calibrated by the factory to read say 60 MPH on a 120 speedo and the same on a 100 speedo.
It is the drive pinion in the tranny that feeds the information to the speedometer.
The factory recommended drive pinion used is determined by the actual speed of the car if the gearing and rear tire diameter is stock.
Any change to either will necessitate the use of a speedometer pinion that actually matches the speed of the car as it is presently set up.