You can get ratio adapters that fit into the speedo cable - either on the gearbox or the speedo head. They are miniature gearboxes which can be built up to the exact ratio you need. They used to be used often on commercial vehicles to adjust tachograph speeds in the pre-digital age, but I believe they are still available. Hot-rod places might be a good place to start looking.
The link I put up does that, supplies a ratio adapter that fits into the gearbox.
