Changing '97 Sierra rear gear ratio

['ol 47]

Hi, I just picked up a '97 GMC Sierra 1500, short box, regular cab, with th 4.3. It's fine, and in great condition, but the rear gears are 3.08 and I want to change them to 3.42, for towing our small(20ft) trailer, and because we have a lot of steep, but short, hills around these parts. Couple questions:


a) What is involved in re-calibrating the speedometer after the swap?


b) I have found a few diffs in wrecking yards with the right gears, but none are from a '97. What years interchage directly? I want to just change the pinion and entire carrier, not the whole rear end housing...


c) Is there anything else I need to know, or look out for?

thanks!

[maxwedge]

Any salvage yard can tell from the Hollander guide just what will fit. The pcm will have to be reprogrammed for the speedo calibration. A good shop can put a 3.42 ring and pinion in for you, of course this takes some experience and the proper tools to do this right. The diff. should never be swapped into another carrier ,setting up the preload and backlash can be problematic as the tolerances for each one vary slightly. The preload shims are matched to the housing and carrier.

['ol 47]

Yes, I know of the guides, but the shady characters around here are the ones I don't trust. One says they are identical from 88-96, another says they're all the same....


I'm aware of the shimming involved, checking the pattern, etc...; been there, done that before. I've been out of the loop, so to say, for over a decade and a half, so it's the 'what fits what' that i'm not up to date with. The "mechanics" of it all I should be ale to still handle.


What part of the ECM gets worked on? Is it a chip I can swap, or is it a dealer re-programming deal?? I live in a remote area of Northern Ontario, and we don't have specialty shops.


Thanks!

[maxwedge]

A dealer scanner can reprogram the pcm software no chip or prom in your case. Mitchell guide shows 92-01 c/k truck uses the same housing.

['ol 47]

Thanks!

I'm off(like a new bride's pyjamas.....) and shopping!