I have been really lazy and havent really asked this question, I have changed the whole running gear in my car to JDM, and the only thing that broke was the speedo cable. I put my ADM one back in and the speedo is up the shit, (wobbles at slow speeds, Its about 10km/h under the actual speed). Whats the best option? Should I just buy a new JDM one or is there a converter that you can buy?

Would a gearbox shop be able to fix this problem??

Cheers