3.73s with 35s 72 K10 Th350 Auto & stock 350?

[fishmunger]

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...

[57taskforce]

With the weight your wanting to tow I think I’d stick with 4:11’s. If you had a 4 speed with granny to get you out of the hole, 3.73’s might be an ok option. I just don’t think the 3.73 would be ideal for towing with 33’s or 35’s, second gear might line up but it’s gonna be a dog off the line with a trailer behind it... That’s just my opinion though.

[special-K]

There is very little difference between the two ratios. To me the one with the most advantage for your uses would be 4.10s, especially considering your taller than factory tires. With 35s and 4.10s on a truck that originally had 31" tires you end up with right about 3.73 ratio. I wouldn't want less than that to tow or haul weight

[Jcfcamaro]

I'm getting ready to do the same with my 72 k10 with the same drive trane. Doing all the transmission calculators online it seems like the 3.73 is a good fit for me, but I'm going to see what the guys on here say. When you get ready to order parts check out Quick Performance out of Ames Iowa they have really good prices and a great tech line for help. Also sell package kits . That's where I'll be buying my stuff.

[mongocanfly]

what gears do you have now? must be fairly low
and what rpm do you want to turn at say 65mph?
the gears you stated are gonna give the rpms your saying are to high..
3.73-33.5-1:1-65mph=2432rpm
4.11-33.5-1:1-65mph=2679rpm
I would look more towards a od trans with a lower axle ratio to get the best of both worlds
hers the calc I use ..just fill in blanks at bottom of the page..
http://www.csgnetwork.com/multirpmcalc.html

[cadillac_al]

My 76 K10 350/th350/30"tires tows 6000 lb trailers with 3.08 gears and I don't need to downshift for hills. I wouldn't worry about the downshifting aspect.

[fishmunger]

Currently the truck has 4.56s, and it's in 3rd gear at about 15 mph going down the road! Takes 3000 rpms to go 65, even at 2500 its barely pulling 55 and I'm practically getting run off the highway...

[fishmunger]

cadillac al - Are you satisfied with your setup for towing? How is your off the line response? Your effective ratio is pretty similar, actually slightly higher, than what I would be looking at with 3.73s

[1976gmc20]

Originally Posted by cadillac_al

My 76 K10 350/th350/30"tires tows 6000 lb trailers with 3.08 gears and I don't need to downshift for hills. I wouldn't worry about the downshifting aspect.

Must not be very steep hills. My old 350/350 K trucks wouldn't even climb the hills in Colorado in high gear when empty

And one I bought brand new. The other was in the family since new and I got it at about 12 years old, maybe 65K miles.

Maybe altitude makes a difference ???

[Mike C]

3.73 and 35" tires are a great choice with a TH350. Same setup we did in my kids Jimmy. And if you had a manual trans you'd probably like the 4.56.

Based on your description of use, I'd go with the 3.73. On my kids truck we also did a 204/214 cam with an Edelbrock intake and dual exhaust on 2.5" rams horns so a little more power than stock.

Converter slippage is a factor making 3.73 and a TH350 almost same highway rpm as an sm465 and 4.10 which is my favorite road combo.

[Zoomad75]

If hills or mountain grades are in the plans for driving, I'd error on the side of more gear.

I've lived with 350/465/3.73 on 35's here in Colorado. Flat ground it was ok, but anytime the grade went up the poor little 350 was struggling to maintain speed.

[4u2nv]

Originally Posted by fishmunger

Currently the truck has 4.56s, and it's in 3rd gear at about 15 mph going down the road! Takes 3000 rpms to go 65, even at 2500 its barely pulling 55 and I'm practically getting run off the highway...

That's the rpm I'm getting at the same speed with 4.11's and 35's , It's off the line like a rapped ape and through 2nd in a flash , I shouldn't think that there is any thing to worry about for towing except your gas mileage.

[57taskforce]

Originally Posted by 4u2nv

That's the rpm I'm getting at the same speed with 4.11's and 35's , It's off the line like a rapped ape and through 2nd in a flash , I shouldn't think that there is any thing to worry about for towing except your gas mileage.

Are you sure your rpms are the same? Mine has 4:11’s, Sm465 with 1:1 4th gear, and 31.5”ish tires on it and it doesn’t quite pull 3K@65. With 4:11’s and 35’s
You should be closer to 2500@65, and 2100@55. That’s of course assuming a 1:1 transmission high gear.

[fishmunger]

Thank you for all the responses!

Mike C - I'm curious, did your kid's rig see any towing?

[1976gmc20]

Originally Posted by Zoomad75

If hills or mountain grades are in the plans for driving, I'd error on the side of more gear.

I've lived with 350/465/3.73 on 35's here in Colorado. Flat ground it was ok, but anytime the grade went up the poor little 350 was struggling to maintain speed.

I once had a 350/465/3.07 and oddly enough it did all right. Pulling my trailer up a hill you could just shift down to 3rd and keep right on going. Of course the speed limit was 55 in those days but it would run 55 in 3rd without any complaint. Solo it would get 18 mpg at 55 and 15 mpg at 80. You pretty much had to leave it in 3rd driving around town; 4th(H) was effectively overdrive.

Pulling my Cat on a rented trailer over Monarch, it did just fine in 2nd gear at 30-35 mph. Coming down was frightful though! I never did that again. I bought a Real Truck and had to sell the K10 to pay the bills.

[4u2nv]

Originally Posted by 57taskforce

Are you sure your rpms are the same? Mine has 4:11’s, Sm465 with 1:1 4th gear, and 31.5”ish tires on it and it doesn’t quite pull 3K@65. With 4:11’s and 35’s
You should be closer to 2500@65, and 2100@55. That’s of course assuming a 1:1 transmission high gear.

350 turbo tranny

[Mike C]

Originally Posted by fishmunger

Thank you for all the responses!

Mike C - I'm curious, did your kid's rig see any towing?

It has not seen any towing, but I don't see any reason it wouldn't do well at it.

[i82much]

I have a 1972 K10 with 3.73s, 32 inch tires, TH350, and a 396. It’s pretty good all-around but if I had to open up the axles for some reason I would swap in a little steeper gear like 4.11.

It’s hard for me to see how a 350 would do any better than my 396, so my guess is you’d want more gear than 3.73?