My GT6 came to me with a different rear-end than stock and required one of these ratio-conversion-boxes to match the speedometer to the final drive. At the time, 15 years ago, it cost me about $80 and I had to provide the speedometer shop two (2) new speedo cables and a marked up drawing showing where (inline) I wanted the ratio box to be mounted. It worked fine and would still be in service today but I eventually put a standard rear end back in the car which allowed me to remove the ratio-box and put in a standard speedometer cable.