The 1947 - Present Chevrolet & GMC Truck Message Board Network







Register or Log In To remove these advertisements.

Go Back   The 1947 - Present Chevrolet & GMC Truck Message Board Network > 47 - Current classic GM Trucks > The 1973 - 1987 Chevrolet & GMC Squarebody Pickups Message Board

Web 67-72chevytrucks.com


Reply
 
Thread Tools Display Modes
Old 10-15-2016, 07:41 PM   #1
learyw
Registered User
 
learyw's Avatar
 
Join Date: Dec 2015
Location: Westminster, CO
Posts: 26
Swapping 4.56 to 3.73 or maybe 4.10

I run my 78 K20 stock. I carry lumber/tools in the bed, but I rarely/never tow. I have 4.56 gearing in the rear. I get terrible gas milage. Has anyone swapped to a 4.10? How much better was your milage? I know my GM 10.5" 14 bolt only came in 4.10 or 4.56 originally. Can you go down to a 3.73? Thanks!
__________________
1978 K20 Camper Special, stock 350, TH400, np203, 265/75R16, GMC 10.5" rear, Dana 44 front 3.73, Q-jet.
learyw is offline   Reply With Quote
Old 10-15-2016, 10:26 PM   #2
wilkin250r
Registered User
 
wilkin250r's Avatar
 
Join Date: Sep 2010
Location: Lake Tahoe, Nevada
Posts: 755
Re: Swapping 4.56 to 3.73 or maybe 4.10

What size tires are you running?
__________________
I know a little about cars, but if you have a question about electricity or sport quads, I'm your man!!!
wilkin250r is offline   Reply With Quote
Old 10-15-2016, 11:50 PM   #3
learyw
Registered User
 
learyw's Avatar
 
Join Date: Dec 2015
Location: Westminster, CO
Posts: 26
Re: Swapping 4.56 to 3.73 or maybe 4.10

265/75R16. Pretty close to the stock size.
__________________
1978 K20 Camper Special, stock 350, TH400, np203, 265/75R16, GMC 10.5" rear, Dana 44 front 3.73, Q-jet.
learyw is offline   Reply With Quote
Old 10-16-2016, 01:42 PM   #4
Alex V.
Registered User
 
Alex V.'s Avatar
 
Join Date: Mar 2013
Location: Campbellsville, KY
Posts: 888
Re: Swapping 4.56 to 3.73 or maybe 4.10

For the 20/30 series trucks w/16" rims (assuming close to stock tires) IMHO 4.10's are the best all-purpose ratio. On my '85 with stock 454/TH400/4.10's, the only reason I'd go down to 3.73's is if I was going to use it primarily for highway towing. Otherwise, 4.10's are a good compromise providing adequate RPM's to accelerate and climb hills in <55 MPH speed limits, but keeping RPM's below 3,000 until around 70 MPH.
__________________
Alex V.
------
1967 C10 Suburban, 350/NP435, Green/Green, PS, PB, HD cooling, charging, shocks, and springs.

1985 GMC C3500 SRW, Sierra Classic, 454/TH400, white/blue.
Alex V. is offline   Reply With Quote
Old 10-16-2016, 09:42 PM   #5
Big Port Jimmy 6
Addicted to Rust
 
Big Port Jimmy 6's Avatar
 
Join Date: Dec 2009
Location: Abbeville SC
Posts: 1,030
Re: Swapping 4.56 to 3.73 or maybe 4.10

4L80E or gear vendors swap? Makes a 4.56 into a 3.32. Although you can pay for a lot of gas for the cost. Jay
__________________
Current trucks:
(48 GMC100 320 inliner) Gone
(48 Suburban in pieces) Sold
87 GMC CCSB 5.3 4l60E Daily Driver

ECTA National record holder XO/PP

We never really grow up, we just learn how to act in public.
Big Port Jimmy 6 is offline   Reply With Quote
Old 10-16-2016, 10:21 PM   #6
cochino12
It followed me home?
 
cochino12's Avatar
 
Join Date: Mar 2003
Location: Yup
Posts: 5,751
Re: Swapping 4.56 to 3.73 or maybe 4.10

That would be a lot of miles to justify the cost of a gear swap. If it is truly that big of an issue I'd buy bigger tires instead.
__________________
Adam

That's why they call it a shortcut Kyle, if it was easy it would just be the way.

86 c-10 SWB
85 CCSB
90 GMC K30 Crew SRW
1985 C30 Dually
cochino12 is offline   Reply With Quote
Old 10-17-2016, 06:29 AM   #7
D13
Registered User
 
D13's Avatar
 
Join Date: Feb 2007
Location: Memphis MI
Posts: 1,851
Re: Swapping 4.56 to 3.73 or maybe 4.10

My 81 factory light 3/4 ton has 3.43's with no overdrive. My 99 weighs 5600# with me and no gas and has 3.43's with OD, running LT235/16/r78's. With an overdrive trans and a v8, 3.73's would probably be better. You can get away with less gears with a V8 and an auto due to torque multiplication in the converter for starting the load.

Best bet is to pick your tire and trans, then use an online calculator to find the gear. I like to run my stuff 17-1800 at 60mph, puts 72 right around 2000 rpm which should be in the good part of the torque curve. I don't tow a ton (mostly DD) and when I do I run in 3rd gear and keep the speeds low.
__________________
1987 2 ton
1982 250/TH350 beater in progress
Dad's 1981 3/4 L6 3 on tree posi and no options, awaiting restoration or scrapping
Plus a mess o' tractors
D13 is offline   Reply With Quote
Old 10-18-2016, 12:36 PM   #8
DieselSJ
Registered User
 
Join Date: Jun 2016
Location: Phoenix, AZ
Posts: 574
Re: Swapping 4.56 to 3.73 or maybe 4.10

IMHO, 3.73 would be fine for what you do. I have 4.10 in my CC dually and really wish I had an overdrive on the freeway. For around town it is even a bit steep. I think I would be happy with 3.73 for 95% of what I do. I would be happiest with 4.10 and overdrive.
DieselSJ is offline   Reply With Quote
Reply

Bookmarks

Tags
differential, gearing

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 04:40 AM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Copyright 1997-2022 67-72chevytrucks.com