changing gear ratio

That is because you need to find a truck shop, one that deals in big trucks, the dealer will just waste your time, and your local mechanic that works on cars doesnt have the equipment to get the job done.

 

If you don't already have a ratio adapter you can change the gears in, you will have to get one at a place that knows what gear sets will get your speedo right. You may have to call around a bit since electric speedos have taken over. It used to be all trucks were cable drive, and most truck dealers had adapters and gears in stock.

Ratio Adapter:

 

Find a used truck salvage yard and poke around... maybe you'll get lucky. But you SHOULD be able to get the right parts from a KNOWLEDGEABLE parts guy...