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Old 10-15-2016, 07:41 PM   #1
learyw
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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!
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Old 10-15-2016, 10:26 PM   #2
wilkin250r
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Re: Swapping 4.56 to 3.73 or maybe 4.10

What size tires are you running?
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Old 10-15-2016, 11:50 PM   #3
learyw
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Re: Swapping 4.56 to 3.73 or maybe 4.10

265/75R16. Pretty close to the stock size.
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Old 10-16-2016, 01:42 PM   #4
Alex V.
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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.
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Old 10-16-2016, 09:42 PM   #5
Big Port Jimmy 6
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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
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Old 10-16-2016, 10:21 PM   #6
cochino12
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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.
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Old 10-17-2016, 06:29 AM   #7
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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.
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Old 10-18-2016, 12:36 PM   #8
DieselSJ
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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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