Frequency adjuster circuit for electronic speedometer

[Pet]

Hi all, I'm doing freq. adjuster for my electronic speedo (Opel Calibra 260km/h type) - when I hook ABS wheel speed signal direct to that speedo, it's working, but there is speed error aprox. 17%. (50Km/h real - tacho shows 60km/h..) Therefore I am creating simple circuit with PIC18 MCU, which allows adjust frequency in range +/- 50% - 1% steps (control via trimmer hooked to AD) (+ multiply/divide by 2x,4x - via jumpers). Input/Output Frequency can be between 40 - 10000Hz. Software is now working well, I am preparing the test in car.
If somebody is interest about that stuff, let me know, if not drink one beer  :D
(if there will be more people I can create small SMD PCB's for better price...)

cheers