speedometer recalibration

[1990 GMC]

I have lifted my GMC and put 33's on it. I bought a tru speed recalibrator for the computer, but the instructions only go back as far as 1990 and they are really general. I was hoping maybe somebody could tell me exatly which wire it is that sends the signal to the ecm so that I can possibly get the recalibratior to work. I tried splicing into the two wires on the sensor but that didn't work either. HELP!

[maroonsilverado]

i may be totally off with this, but here goes. your truck should have the 700R4, and you can't recalibrate it that easily. most of the plug-in recalibraters i've seen are for the 4L60E...which is an electronic shifing tranny. but i could be wrong

[snoman]

Actually it is real easy and more flexable than the ECM based ones. Get under your truck and on transfer case output housing there it a bolt holding speedo drive gear assembly in. Remove bolt and take it out and count the teeth on the gear (it can be from about 34 to 47 or so). Your will probaly be around 43 or so. Use one less tooth on drive gear for each for each 3mph that it reads slow at 60mph. It your case I am guesing around two teeth less is needed. GM dealers either stock or can order correct size gear for you. I have several of them myself from past tweaks.