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That speedometer (oval) was used until about July 1930. At that time the speedometer was changed to round. If your dash plate opening requires an oval speedometer either the fuel tank has been changed or the car is not a 1931.
To get the correct speedometer drive housing and gear you must determine the differential gear ratio. Most passenger cars of this year were 3:78 to 1 ratio.
One way to determine the ratio is:
Remove the spark plugs to make turning the engine easier.
Place a mark on the crankshaft pulley and a corresponding mark to check revolutions.
Place the transmission in 3rd gear.
Place a mark on the bottom of the right front tire at the floor.
Using the right front tire, pull the car forward observing and counting the engine turns until the tire mark is on the floor.
If the turns are just over 3 1/2 it is a 3:54 to 1
If it is close to 3 3/4 turns it is a 3:78 to 1
If over 4 turns it is a 4:11 to one.
Vendors sell the proper Adapter for the correct ratio.