Find a gear that fits and has the correct helix on the gears. Anything will do. Take it for a drive, figure out how much off you are. (I prefer using a GPS for the accurate speed). Calculate the percentage that the speedo is off (60 indicated is really 66 on GPS the you are 10% slow) Multiply the error percentage to the number of teeth on the speedo gear. This is the correction number. (example is 18 teeth multiplied by 10% = 1.8 correction)
If the speedo reads slow, subtract the correction number from the number of teeth (18-1.8=16.2) and round to the nearest whole number (16 tooth).
If the speedo reads fast, add the correction number to the number of teeth (18+1.8=19.8) round to 20.
If you are at half a tooth, remember that less teeth makes the speedo read faster, thus less ticket prone (But Officer I was only doing 60, but radar says you were doing 58 in a 35… )
