3.73s with 35s 72 K10 Th350 Auto & stock 350? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

fishmunger · 02-02-2021, 05:31 PM

Finally got my project K10 on the road! It currently has 35s (small ones - actual diameter 33.5 or so), th350 auto, stock 350 that runs good, and it just sings going down the highway!

It's going to be my "backup" work truck which means it will probably see 3-4k miles a year, and 50% of those will be towing a smallish, (20' - 4000# total) boat w/ trailer (I'm in the fishing business). It will very occasionally have to tow a 6000lb boat/trailer, but those times will be few and far between, and mostly short distances (the overall size of this truck is simply too small to tow a load that size regularly/long distance in my opinion. Truck will also see some offroad trails throughout the year, but nothing crazy, no rock crawling type stuff.

Most of my commuting and towing is on the highway, fairly flat, only one good grade I encounter with some regularity that will likely require a downshift.

Currently its takes 2500-3000 rpms to go 55-65 down the highway, way too high. I was originally leaning towards 4.11s, which seems like a pretty common setup for this combo, and would drop my rpms by about ~300 at a given speed.

But after talking to a very knowledgeable local mechanic who has a 72k5 of his own, and works on lots of 4x4s, he thinks I'd be better off with 3.73s. His argument is that would put my rpms right where I want it most of the time, 2000-2500 on the hwy. And would make 2nd gear 'perfect' when I go up a big grade - just downshift a gear. He thinks 4.11s would be OK, but would be just a 'little bit too low' almost all the time - and the 2nd gear would be too low to downshift into on the hwy. Whereas, the 3.73s would only be a 'little bit high' occasionally. And for the trails, I can just use 4 low if it's a tad sluggish.

I think his points are pretty interesting? I hadn't thought about the downshift into 2nd factor...

Just wanted to get as many opinions as possible before I drop $3500 regearing and rebuilding both axles with new gears and trutracs front and rear...

02-02-2021, 05:31 PM	#1
fishmunger Registered User Join Date: Jan 2005 Location: CA Posts: 411	3.73s with 35s 72 K10 Th350 Auto & stock 350? Finally got my project K10 on the road! It currently has 35s (small ones - actual diameter 33.5 or so), th350 auto, stock 350 that runs good, and it just sings going down the highway! It's going to be my "backup" work truck which means it will probably see 3-4k miles a year, and 50% of those will be towing a smallish, (20' - 4000# total) boat w/ trailer (I'm in the fishing business). It will very occasionally have to tow a 6000lb boat/trailer, but those times will be few and far between, and mostly short distances (the overall size of this truck is simply too small to tow a load that size regularly/long distance in my opinion. Truck will also see some offroad trails throughout the year, but nothing crazy, no rock crawling type stuff. Most of my commuting and towing is on the highway, fairly flat, only one good grade I encounter with some regularity that will likely require a downshift. Currently its takes 2500-3000 rpms to go 55-65 down the highway, way too high. I was originally leaning towards 4.11s, which seems like a pretty common setup for this combo, and would drop my rpms by about ~300 at a given speed. But after talking to a very knowledgeable local mechanic who has a 72k5 of his own, and works on lots of 4x4s, he thinks I'd be better off with 3.73s. His argument is that would put my rpms right where I want it most of the time, 2000-2500 on the hwy. And would make 2nd gear 'perfect' when I go up a big grade - just downshift a gear. He thinks 4.11s would be OK, but would be just a 'little bit too low' almost all the time - and the 2nd gear would be too low to downshift into on the hwy. Whereas, the 3.73s would only be a 'little bit high' occasionally. And for the trails, I can just use 4 low if it's a tad sluggish. I think his points are pretty interesting? I hadn't thought about the downshift into 2nd factor... Just wanted to get as many opinions as possible before I drop $3500 regearing and rebuilding both axles with new gears and trutracs front and rear...



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