The encoding used a prime number as a factor of the number reported, i.e. the ordinal number of each option is multiplied by that prime factor. The prime was selected as the largest possible number based on the max digits to send and the number of options to encode. That created a sparse distribution of possibilities, so that typos were more probable to get detected. It also enabled that by simply factoring the report and using the factors to identify the type of wheel that was sending the reports, allowing one single phone number to receive reports from multiple wheel types or versions.