4wd C20, 4.11 to 3.42?

[CSFJ]

It'll accept a 3.42, 3.08, and 2.73 ring and pinion. Remember that what you do to the rear differential has to be done to the front one. It might have a 700r4, but considering that it's a K20, it'll probably have a three speed. You can swap that in there, but you might consider installing a gear vendor so you can retain the more solid transmission, add gears, and not have to modify anything or add the stand alone computer if you had a 4l80e in mind. Bear in mind that if you swap in those types of gears, it does improve the economy, but it won't tow anything, and it renders those beefier 3/4 ton axles pointless. Unless the truck is pristine, you might consider a K10 that already has a 700R4, smaller axles, and four wheel drive. Then you can swap the gears on both ends and potentially save money. Just my two cents.

Being a 3/4 ton doesn't automatically guarantee a three speed trans. Sometime during the 80's they started offering the 3/4 in what I guess you could call light duty (700r4, 14 bolt semi float rear axle) and heavy duty (th400, 14 bolt full float rear axle).

 

[1987 GMC Jimmy]

I gotcha. I knew that it was possible to have both, but I thought it was a lot more likely to have the three speed. My bad.

 

[Stepperado]

After speaking with Randy's Ring and Pinion, it looks like the longest gear my carriers will accept is a 3.42. That wouldn't buy me much in mpg gain.

Looks like a 700-R4 is probably my best bet. I don't really haul anything heavy, so the weaker trans doesn't bother me much. I just have occasional loads of dirt or mulch for the house and trips to Home Depot.

Anything tricky to look out for when going from a TH350 to the 700-R4?