You can buy a mechanical adapter from a variety of places. It costs between $70-$100. If you are interested, I'll look for links and post them.
I have used an ($100) adapter and my speedometer is within 1/2% of accurate. The adapter that I bought is tuned to match my larger diameter tires and the 3.55 gear-set. It bolts right into the transmission and then the speedometer cable bolts into the adapter.
Tim
