Speedometer setup help needed

Hey All,
Had a new transmission put in my 71 Demon, upgrading from a 904 transmission to the 727.
My speedometer is now out about 10 MPH high at 40 MPH.
So, thinking either the internal speedometer gear of the transmission is a different tooth count or the car shop that changed out the transmission did not convert the speedometer gear over to the new 727.
So, how do i figure out the correct gear teeth count needed ?
Driveline is a 340 with extras, 3200 stall 727 with shift kit, 3:91 rear end and P275/60R15's on the back.
Any help would be appreciated.
Thanks Mike