differential gearing - The 1947 - Present Chevrolet & GMC Truck Message Board Network

WyattEarp18 · 01-24-2016, 06:26 AM

I've got a 79 k20 camper special 350/th400/np205. Before I converted to the 205 my max speed was around 60-65 without worrying about blowing the motor. After the swap I'm down to around 50-55. The 205 came out of a 80 k30. Just wondering what my rear gear ratio is? I read they came with 4.10 or 4.56 rears. Reason I ask is im looking at adding 4-6" of lift and 33 or 35s on it. I'd rather not replace the diffs but I'd also like to be able to do 70 on the highway. Any help is greatly appreciated.

obijuan · 01-24-2016, 10:18 AM

Just a tcase swap will have no effect on the rpm.
But to be sure, take to diff cover off the rear and read the numbers on the ring gear. They will tell the ratio. Then you can determine where your rpms should be when. Then determine if you have torque converter issues or something else.

**Zoomad75** · 01-24-2016, 12:11 PM

Keep this in mind. Bigger tires effectively need more gear to get rolling. One thing some forget is the bigger tires have an effect like an overdrive gear. Effectively for each revolution of the tires, you are going further than you would with a smaller tire. So if you already have 4.10's already you will be ok with either 33's or 35's. 4.56's with the 35's is the sweet spot if you got them.

Still, like Obijuan said, pull the cover and read the numbers on the ring gear. You'll know for sure. I've found the 4.10 ratio is more common on the K20 vs the 4.56 gear, 3.73's are probably the most common.

WyattEarp18 · 01-24-2016, 07:56 PM

Where will the numbers be on the ring, the outer edge I assume. I'm guessing that the 205 I swapped in has a lower output gear ratio than my old 203. Torque converter is brand new along with a quality rebuilt transmission. Thanks for the help guys

**Stocker** · 01-24-2016, 08:17 PM

With the diff cover removed, don't worry if you don't find the numbers. Just count the teeth on the ring gear and divide by the number of teeth on the pinion gear.

As to your two t-cases, low range is different but high range is the same at 1:1.

**JIMs70GMC** · 01-24-2016, 08:26 PM

I would check your speed against a GPS unit, I bet the speedo gear is different between the two swapped transfer cases.

WyattEarp18 · 01-24-2016, 08:32 PM

That's how I got the speeds. The speedo adapter took a dive on me so I've based my speed off 3 different speedometer apps. I'll check the diff this week. Thanks for the help

bilfman · 01-24-2016, 11:52 PM

Definitely not the tcase for the difference. Unless you are in low range. But it could be the speedo drive gear is different .

01-24-2016, 06:26 AM	#1
WyattEarp18 Registered User Join Date: Mar 2014 Location: Elko Nevada Posts: 43	differential gearing I've got a 79 k20 camper special 350/th400/np205. Before I converted to the 205 my max speed was around 60-65 without worrying about blowing the motor. After the swap I'm down to around 50-55. The 205 came out of a 80 k30. Just wondering what my rear gear ratio is? I read they came with 4.10 or 4.56 rears. Reason I ask is im looking at adding 4-6" of lift and 33 or 35s on it. I'd rather not replace the diffs but I'd also like to be able to do 70 on the highway. Any help is greatly appreciated. __________________ '79 K20 Camper Deluxe '99 F-250 PowerStroke '98 Z71 4x4 Some people are like slinkies, good for nothing but make you laugh when tossed down a flight of stairs.

01-24-2016, 10:18 AM	#2
obijuan Registered User Join Date: Jun 2012 Location: cibolo tx Posts: 1,033	Re: differential gearing Just a tcase swap will have no effect on the rpm. But to be sure, take to diff cover off the rear and read the numbers on the ring gear. They will tell the ratio. Then you can determine where your rpms should be when. Then determine if you have torque converter issues or something else.

01-24-2016, 12:11 PM	#3
Zoomad75 K5Camper Join Date: Mar 2004 Location: Pueblo, CO Posts: 1,513	Re: differential gearing Keep this in mind. Bigger tires effectively need more gear to get rolling. One thing some forget is the bigger tires have an effect like an overdrive gear. Effectively for each revolution of the tires, you are going further than you would with a smaller tire. So if you already have 4.10's already you will be ok with either 33's or 35's. 4.56's with the 35's is the sweet spot if you got them. Still, like Obijuan said, pull the cover and read the numbers on the ring gear. You'll know for sure. I've found the 4.10 ratio is more common on the K20 vs the 4.56 gear, 3.73's are probably the most common. __________________ 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

01-24-2016, 07:56 PM	#4
WyattEarp18 Registered User Join Date: Mar 2014 Location: Elko Nevada Posts: 43	Re: differential gearing Where will the numbers be on the ring, the outer edge I assume. I'm guessing that the 205 I swapped in has a lower output gear ratio than my old 203. Torque converter is brand new along with a quality rebuilt transmission. Thanks for the help guys __________________ '79 K20 Camper Deluxe '99 F-250 PowerStroke '98 Z71 4x4 Some people are like slinkies, good for nothing but make you laugh when tossed down a flight of stairs.

01-24-2016, 08:17 PM	#5
Stocker 20' Daredevil (Ret) Join Date: May 2003 Location: Jefferson State Posts: 13,525	Re: differential gearing With the diff cover removed, don't worry if you don't find the numbers. Just count the teeth on the ring gear and divide by the number of teeth on the pinion gear. As to your two t-cases, low range is different but high range is the same at 1:1. __________________ - Mike - 1972 K20 LWB 350/350/205 RIP El Jay

01-24-2016, 08:26 PM	#6
JIMs70GMC user # 2756 Join Date: Jun 2001 Location: Chesapeake, Virginia Posts: 4,612	Re: differential gearing I would check your speed against a GPS unit, I bet the speedo gear is different between the two swapped transfer cases. __________________ 1970 K25, 8' stepside bed 350/465/205 44 up front, 60 in the rear 4.10s rolling on 33" Dunlop MTs 1986 K5, 350/465/208 Dana 60/14 bolt from a cucv 36" Super Swampers TSL/SX 1983 K20 w/ CUCV axles, 350/700R4/208 sitting on 37" Goodyears 1986 M1031 6.2 diesel, TH400/NP205 locker in the rear and a LS in the front, all stock for now..... 1986 K30, 350/400/205 dana 60 and 14 bolt. I kept the drivetrain. Body/bad and chassis are gone. 1981 K30, 350/465/205 dana 60 and dually 14 bolt. Has a G80, and a flat bed. Going to replace the flat bed. 1985 K20, 350/400/208 10 bolt and SF 14 bolt. I wonder where I can find some 1 tons. Hmmmmm

01-24-2016, 08:32 PM	#7
WyattEarp18 Registered User Join Date: Mar 2014 Location: Elko Nevada Posts: 43	Re: differential gearing That's how I got the speeds. The speedo adapter took a dive on me so I've based my speed off 3 different speedometer apps. I'll check the diff this week. Thanks for the help __________________ '79 K20 Camper Deluxe '99 F-250 PowerStroke '98 Z71 4x4 Some people are like slinkies, good for nothing but make you laugh when tossed down a flight of stairs.

01-24-2016, 11:52 PM	#8
bilfman Registered User Join Date: Dec 2011 Location: Regina, SK, Canada Posts: 622	Re: differential gearing Definitely not the tcase for the difference. Unless you are in low range. But it could be the speedo drive gear is different . __________________ 1979 Chevy Dually Crew 4X4 454, D60 Tru-trac, D70 Detroit Locker, 3.73, 4L80E/NP205 1971 GMC 3/4 Ton 4X4



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