Speedo Math

["JOHN JOHNSON" <john70350@msn.com>]

I am about to install a Volvo converted J-type overdrive into a 1974 TR6, 
replacing a 4 speed. The tires on the car a shorter than stock so the speedo 
has always been fast. I don't need it to be totally correct but I would like 
to err on the slow side if possible. The overdrive has a 20 tooth driven 
gear and I don't know know how many teeth the drive gear is. Are Volvo 
overdrives speedo gears the same? If not any calibration ideas? Thanks.