Hmmm,
I'm a little confused. I thought if you had a DRAC, then you did have an electric speedo. If not, you can play around with speedo gears and reducer housings to get the correct reading. See you GM parts dealer and ask him to print the page for your year truck listing all the speedo gear combinations relating to tire size and gears. You'll have to do some math as GM never figured for tires larger than 33". There should be a listing of reducer gear housings as well (1:0 to 1:33 ratio or something like that), they can help out to. These fasten to the transfer case and the speedo cable fits to it. There are usually 2 different gears that can fit in the transfer case that drive the speedo gear. Pay attention to their color (they are identified by color) as the speedo gears can only mesh with one or the other.
If it is an electronic speedo and you do have a DRAC under your dash, call Technicar 1-800 222-5509 and tell them your tire size and gears and they'll burn you the correct DRAC for around $90.
Eric M.
