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#1 |
Registered User
Join Date: Jan 2005
Location: CA
Posts: 411
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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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#2 |
All about them K’s
![]() Join Date: Jun 2014
Location: Land of Entrapment
Posts: 6,401
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Re: 3.73s with 35s 72 K10 Th350 Auto & stock 350?
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.
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Tyler '57 3100 http://67-72chevytrucks.com/vboard/s...d.php?t=813888 '72 K20 Cheyenne: 5” lift, 35’s, front dana 60 blah blah blah… http://67-72chevytrucks.com/vboard/s...d.php?t=662879 ‘69 K10 SWB: 4” lift 33”s… in a million pieces http://67-72chevytrucks.com/vboard/s...d.php?t=805206 '98 Silverado LT K2500HD ECLB Vortec 454/4l80E: 6" lift 35x12.5x20’s |
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#3 |
Special Order
![]() ![]() Join Date: Nov 2004
Location: Mt Airy, MD
Posts: 85,858
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Re: 3.73s with 35s 72 K10 Th350 Auto & stock 350?
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
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"BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~ |
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#4 |
Senior Member
![]() Join Date: Jul 2015
Location: Fairfax, Missouri
Posts: 1,049
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Re: 3.73s with 35s 72 K10 Th350 Auto & stock 350?
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.
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#5 |
Post Whore
![]() ![]() Join Date: May 2015
Location: Alabama
Posts: 14,652
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Re: 3.73s with 35s 72 K10 Th350 Auto & stock 350?
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
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Mongo...aka Greg RIP Dad RIP Jesse 1981 C30 LQ9 NV4500..http://67-72chevytrucks.com/vboard/s...d.php?t=753598 Mongos AD- LS3 TR6060...http://67-72chevytrucks.com/vboard/s...34#post8522334 Columbus..the 1957 IH 4x4...http://67-72chevytrucks.com/vboard/s...63#post8082563 2023 Chevy Z71..daily driver |
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#6 |
Registered User
Join Date: Oct 2013
Location: Maine
Posts: 2,357
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Re: 3.73s with 35s 72 K10 Th350 Auto & stock 350?
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.
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76 Chevy K20 76 GMC K15 77 Chevy C10 77 Chevy K10 |
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#7 |
Registered User
Join Date: Jan 2005
Location: CA
Posts: 411
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Re: 3.73s with 35s 72 K10 Th350 Auto & stock 350?
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...
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#8 |
Registered User
Join Date: Jan 2005
Location: CA
Posts: 411
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Re: 3.73s with 35s 72 K10 Th350 Auto & stock 350?
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
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#9 | |
Registered User
Join Date: May 2015
Location: Montana
Posts: 3,696
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Re: 3.73s with 35s 72 K10 Th350 Auto & stock 350?
Quote:
![]() 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 ???
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Current/past Chevy/GMC trucks: 1958 Chevy C-60; 1965 GMC C-50; 1965 Chevy C-10; 1971 Chevy K-10; 1973 Chevy K-20; 1976 GMC C-20; 1977 Chevy C-10 Suburban; 1980 Chevy K-10; 1989 Chevy K1500; 1991 GMC V1500 Suburban; 2016 Chevy K2500 HD Other vehicles: 1988 Jeep XJ; 2011 Toyota 4Runner |
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#10 |
Registered User
Join Date: Dec 1999
Location: Austin, TX, USA
Posts: 7,721
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Re: 3.73s with 35s 72 K10 Th350 Auto & stock 350?
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.
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44 Willys MB 52 M38A1 64 Corvette Coupe 68 Camaro 'vert LT1 & TH700 69 Z/28 355 12.6's @110 69 Chevy Short Step 4 1/2"/7" drop 72 Jimmy 4WD 4spd 4" & 35's 02 GMC 2500HD 4x4 Duramax |
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#11 |
K5Camper
![]() Join Date: Mar 2004
Location: Pueblo, CO
Posts: 1,513
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Re: 3.73s with 35s 72 K10 Th350 Auto & stock 350?
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.
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Rob Z. 1975 K5 350/465/205/D44/12b 4" lift on 35's- RIP 1991 K5 8.1L/NV4500/241/D44/14b FWC Camper |
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#12 |
Registered User
Join Date: Oct 2018
Location: Ontario Peterborough
Posts: 765
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Re: 3.73s with 35s 72 K10 Th350 Auto & stock 350?
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.
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#13 | |
All about them K’s
![]() Join Date: Jun 2014
Location: Land of Entrapment
Posts: 6,401
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Re: 3.73s with 35s 72 K10 Th350 Auto & stock 350?
Quote:
You should be closer to 2500@65, and 2100@55. That’s of course assuming a 1:1 transmission high gear.
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Tyler '57 3100 http://67-72chevytrucks.com/vboard/s...d.php?t=813888 '72 K20 Cheyenne: 5” lift, 35’s, front dana 60 blah blah blah… http://67-72chevytrucks.com/vboard/s...d.php?t=662879 ‘69 K10 SWB: 4” lift 33”s… in a million pieces http://67-72chevytrucks.com/vboard/s...d.php?t=805206 '98 Silverado LT K2500HD ECLB Vortec 454/4l80E: 6" lift 35x12.5x20’s |
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#14 |
Registered User
Join Date: Jan 2005
Location: CA
Posts: 411
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Re: 3.73s with 35s 72 K10 Th350 Auto & stock 350?
Thank you for all the responses!
Mike C - I'm curious, did your kid's rig see any towing? |
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#15 | |
Registered User
Join Date: May 2015
Location: Montana
Posts: 3,696
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Re: 3.73s with 35s 72 K10 Th350 Auto & stock 350?
Quote:
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.
__________________
Current/past Chevy/GMC trucks: 1958 Chevy C-60; 1965 GMC C-50; 1965 Chevy C-10; 1971 Chevy K-10; 1973 Chevy K-20; 1976 GMC C-20; 1977 Chevy C-10 Suburban; 1980 Chevy K-10; 1989 Chevy K1500; 1991 GMC V1500 Suburban; 2016 Chevy K2500 HD Other vehicles: 1988 Jeep XJ; 2011 Toyota 4Runner |
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#16 |
Registered User
Join Date: Oct 2018
Location: Ontario Peterborough
Posts: 765
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Re: 3.73s with 35s 72 K10 Th350 Auto & stock 350?
350 turbo tranny
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#17 |
Registered User
Join Date: Dec 1999
Location: Austin, TX, USA
Posts: 7,721
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Re: 3.73s with 35s 72 K10 Th350 Auto & stock 350?
It has not seen any towing, but I don't see any reason it wouldn't do well at it.
__________________
44 Willys MB 52 M38A1 64 Corvette Coupe 68 Camaro 'vert LT1 & TH700 69 Z/28 355 12.6's @110 69 Chevy Short Step 4 1/2"/7" drop 72 Jimmy 4WD 4spd 4" & 35's 02 GMC 2500HD 4x4 Duramax |
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#18 |
Registered User
Join Date: Jul 2008
Location: Bowie, MD
Posts: 313
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Re: 3.73s with 35s 72 K10 Th350 Auto & stock 350?
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? |
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