Ford uses an 8-bit XOR checksum over a specific range. If original checksum was 0x3A , the new one must match the XOR of the modified block. Your mileage calculator must compute this automatically.

100: 12 34 56 78 12 34 56 78 A3 ... Interpretation: BCD bytes 12 34 56 78 = 123,456 km. Checksum A3 = XOR of bytes 0x100-0x107.

Use a programmer (e.g., Xgecu T48, TL866, or an OBD tool like ELS27). Obtain a 16,384-byte binary file.