Swapping 4.56 to 3.73 or maybe 4.10 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

learyw · 10-15-2016, 07:41 PM

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!

wilkin250r · 10-15-2016, 10:26 PM

What size tires are you running?

learyw · 10-15-2016, 11:50 PM

265/75R16. Pretty close to the stock size.

Alex V. · 10-16-2016, 01:42 PM

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.

Big Port Jimmy 6 · 10-16-2016, 09:42 PM

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

cochino12 · 10-16-2016, 10:21 PM

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.

D13 · 10-17-2016, 06:29 AM

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.

DieselSJ · 10-18-2016, 12:36 PM

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.

10-15-2016, 07:41 PM	#1
learyw Registered User 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.

10-15-2016, 10:26 PM	#2
wilkin250r Registered User 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!!!

10-15-2016, 11:50 PM	#3
learyw Registered User 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.

10-16-2016, 01:42 PM	#4
Alex V. Registered User 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.

10-16-2016, 09:42 PM	#5
Big Port Jimmy 6 Addicted to Rust 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.

10-16-2016, 10:21 PM	#6
cochino12 It followed me home? 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

10-17-2016, 06:29 AM	#7
D13 Registered User 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

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.



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