to adjust tachometer on a space star - is it possible ?

Hello,

I wondered if there is a way to correct a tachometer and odometer on a space star 1999 ? I was said it can't be adjusted but there is an error of more than 10 %, and it is not very comfortable when keeping track of consumption, trip mileage, etc. To my knowledge, there is a cable running from the gearbox, but is it supplying all information to the o/b computer ?

Thanks very much in anticipation,

Regards,

Carl