Speedometer running slow

I just bought a Stewart Warner speedometer off of Ebay.Looked really good. I had a Waltham that broke. I installed the Stewart Warner and it runs about 20 miles per hour slow.
As you can see, I have a 1931 ccpu. As you know, they used 30 and 31 year parts on these pickups so I don't really know which drive gear I have.
My question is if I replace the speedo drive gear with a 19 tooth gear,assuming I have a 18 tooth drive gear, will it speed up the speedometer 20 miles per hour?
Thanks for your help.
Ralph in Round Rock
 
No, switching to a 19 tooth from an 18 will slow your reading by about 5%.

How accurate is your odometer reading? If your odo is accurate you might have a weak magnet.
 
I just talk to the guy who rebuilted it. He said the spring on the top was probably to tight. I sent it back to him for ajustment.
. Thanks
Ralph
 
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