One important piece of information which is required is the number of revolutions it takes your speedometer to register one mile. On most modern cars, this may be found on the bottom (or elsewhere) of the speedometer face, usually in very small numbers. Look for something like 940, 960, 1000, or 1020, or something close. This number is the number of speedometer cable revolutions (or equivalent in the case of electronic sender units) to increment the odometer one mile. Most speedometers (modern ones) are 1000 revs/mile, and this is a good starting number to use.
Another vital piece of info is the speedometer drive/driven gear ratios, the gears inside of the transmission or transaxle. The drive gear is the one which is turned by engine power, usually mounted right on the transmission output shaft. The driven gear is the one which is "driven" by the "drive" gear, and is connected to the speedometer cable or electronic sender. The numbers entered in the program are the integer number of teeth - just count 'em up.
The program will compute the best driven gear given a particular drive gear, or the best drive gear given the driven gear value. Remember that there are only set combinations available, which you can find out from the parts store or dealer. Also remember that gear tooth numbers are integer values.
Speedo Revs/mile = (20168*DiffGear*DriveGearTeeth)/(Tdiam*DrivenGearTeeth)Bruce Bowling