I swapped the rear axle on a '49 F-1 with the predictable result. How are you guys correcting your speedometers? Seems to me there are companies making gearboxes that thread to back and correct the reading. Original axle was 4:29, new one is 3:50, 42 mph indicated is 60 actual speed.

 

It depends on the rest of the system. Need to know the trans and speedo info to give you the best approach to bringing it back to correct.

 

Truckee is right, really need to know the set up on transmission and tire size.
If everything is to your liking with regards to those, and speedometer is still not right, and you have a mechanical cable speedometer cable, you have some choices.
For some transmissions, different drive gears in the transmission itself can be changed.
Another option is to to look for a speedometer cable ratio adapter (google) that increases the rotation of the cable.

 

Rule of thumb is 1 SPEEDO cable gear = + or - 5 MPH

https://www.tciauto.com/speedometer-gear-calculator

 

Right, so since you are reading 18 MPH too fast, a driven gear with 3 more teeth than what you have in there now should put you in the ballpark (provided that a gear with that number of teeth exists).

 

I used the site listed below to order the correct gears for my TH350. Not sure if it will work for your transmission.

https://www.tciauto.com/speedometer-gear-calculator