There's an adapter with easily changeable gears that fits between your speedo cable and your tranny. Chevy used 'em on a lot of their 4X4's through the 60's and 70's. Once you hit the wrecking yard and find one,(you might get lucky and find one with the right gears) you can either get replacement gears from a speedo shop or from GM. I have one out in the shop somewhere...
Speed
