How about one of these CORRECTOR SPEEDO MODULE | Car Access Accessories | Car Electronic Access & Accessories | Automotive & Mobile Security | PRODUCTS | AA0376 | Jaycar UK Site
A bloke where I work used one when he changed the engine and speedo in a land rover and his was fine. He used 2 on it actually, as one on its own didn't have enough adjustment to correct it, so he used one feeding into another to bring it down enough.