Low gear or trans or both? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

LilD · 09-28-2010, 11:25 PM

The 70 C20 I just bought revs to the moon at highway speeds. It was a 6 cylinder to begin with but now has a V8. The trans is the original 4speed with the "granny" gear. I was told that the 6 cylinder trucks had a lower gear ratio in the rear end to make up for some low end grunt. So is it the gear in the rear end or the trans that has too low of a gear or a combination? It sucks not being able to enjoy a cruise down the highway and watching the speedo needle and gas gauge needle meet.

Husker · 09-28-2010, 11:38 PM

Probaly the rear end, I have a 6 with 373's and it sucks the fuel going anything ver 50. The transmission in 4th is probaly 1:1.
Ron

treveiger · 09-28-2010, 11:47 PM

Its probably a sm465 with a low rear end and the final gear in the sm465 is 1:1. I have a 69c20 with probably a 4:10 or 4:11 rear end at least and im sure thats what you have if not lower to make up for the v6.

**Stocker** · 09-28-2010, 11:42 PM

Fourth gear is 1:1 ratio, so lacking an overdrive, you can blame the low rear diff gears -- and yes, they are probably quite low on any older 6-cyl 3/4T truck. I'd guess at least 4.10, maybe 4.56, and possibly even lower.

Jacfourteen · 09-29-2010, 12:36 AM

My truck had the 305 v6 originally and it has a granny 4 and 4.56 gears. This combo with 31" tall tires is spinning about 3000 rpm at 60 mph. I plan on swapping to a 3.73 rear gear and an ovedrive tranny (Maybe an NV4500, not sure yet). Then I'd be around 1800 rpm with a little shorter tire at 60 mph.

LilD · 09-29-2010, 10:01 AM

When I went to look at the truck the p.o. told me he thought the rear gears were 4:56. Driving it I would be inclined to agree. Ive read several threads involving swapping rear ends so maybe Ill take that route and kill two birds with one stone. Driver freindly gears and 5 or 6 lugs.

09-28-2010, 11:25 PM	#1
LilD Registered User Join Date: Oct 2009 Location: Tucson, AZ Posts: 186	Low gear or trans or both? The 70 C20 I just bought revs to the moon at highway speeds. It was a 6 cylinder to begin with but now has a V8. The trans is the original 4speed with the "granny" gear. I was told that the 6 cylinder trucks had a lower gear ratio in the rear end to make up for some low end grunt. So is it the gear in the rear end or the trans that has too low of a gear or a combination? It sucks not being able to enjoy a cruise down the highway and watching the speedo needle and gas gauge needle meet.

09-28-2010, 11:38 PM	#2
Husker Registered User Join Date: Dec 1999 Location: Mid NE USA Posts: 2,212	Re: Low gear or trans or both? Probaly the rear end, I have a 6 with 373's and it sucks the fuel going anything ver 50. The transmission in 4th is probaly 1:1. Ron __________________ Member #111 68 C-10 Daily Driver 70 4x4 Project 76 Jeep CJ-5 Help Support the Board: http://67-72chevytrucks.com/vboard/payments.php He who knows nothing is closer to the truth than he whose mind is filled with falsehoods and errors. Thomas Jefferson

09-28-2010, 11:47 PM	#3
treveiger Senior Member Join Date: Jul 2010 Location: South Dakota Posts: 4,093	Re: Low gear or trans or both? Its probably a sm465 with a low rear end and the final gear in the sm465 is 1:1. I have a 69c20 with probably a 4:10 or 4:11 rear end at least and im sure thats what you have if not lower to make up for the v6.

09-28-2010, 11:42 PM	#4
Stocker 20' Daredevil (Ret) Join Date: May 2003 Location: Jefferson State Posts: 13,722	Re: Low gear or trans or both? Fourth gear is 1:1 ratio, so lacking an overdrive, you can blame the low rear diff gears -- and yes, they are probably quite low on any older 6-cyl 3/4T truck. I'd guess at least 4.10, maybe 4.56, and possibly even lower. __________________ - Mike - 1972 K20 LWB 350/350/205 RIP El Jay

09-29-2010, 12:36 AM	#5
Jacfourteen Big Red - Now its a SWB! Join Date: Jun 2009 Location: Oroville, Ca Posts: 1,624	Re: Low gear or trans or both? My truck had the 305 v6 originally and it has a granny 4 and 4.56 gears. This combo with 31" tall tires is spinning about 3000 rpm at 60 mph. I plan on swapping to a 3.73 rear gear and an ovedrive tranny (Maybe an NV4500, not sure yet). Then I'd be around 1800 rpm with a little shorter tire at 60 mph. __________________ 68 3/4 ton (Project)-350 Small Block, .030 over, 10:1 comp, 3998993 heads, C3BX edelbrock intake, comp cams XE268, Holley 670 SA, long tube headers, dual flow 40's, E-fan, Alum rad, currently under the knife getting shortened and bagged build thread 41 3/4 ton (wife's)-flatbed, fully restored to original 01 Tahoe LT 4WD (my daily driver)-Magnaflow Muffler, Rollin on 22's 02 Ford Mustang Convertable (Wife's Car)-Flowmaster 50, Clear corners, Intake, sittin on 18's Last edited by Jacfourteen; 09-29-2010 at 12:36 AM.

09-29-2010, 10:01 AM	#6
LilD Registered User Join Date: Oct 2009 Location: Tucson, AZ Posts: 186	Re: Low gear or trans or both? When I went to look at the truck the p.o. told me he thought the rear gears were 4:56. Driving it I would be inclined to agree. Ive read several threads involving swapping rear ends so maybe Ill take that route and kill two birds with one stone. Driver freindly gears and 5 or 6 lugs.



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