GMC C20 Questions - The 1947 - Present Chevrolet & GMC Truck Message Board Network

lukecp · 03-07-2004, 07:08 PM

My stepbrother has a '70 GMC 3/4 ton longbox he has been trying to sell me for quite some time now. I finally got to look at it and drive it today.
It is an original Colorado truck, so the body is really solid. Needs 2 rockers and a small part of one cab corner, above the windshield, floors, and cab supports are rock solid. Unlike my '72, which needs alot of work in all the commonly rusted areas.

Anyway, it has a coil spring Dana 60 in the rear. It is an orignal 307 truck, with a granny low 4-speed transmission. The original 307 is gone, replaced with a very well running 327 from a 1962 fullsize car (i verfied it with the engine suffix codes).

So, what would the axle ratio in the rear be stock? I'm thinking 4.10, but the truck seemed really low geared so it may be lower. I have the axle ratios for a C10 figured out, but C20s are a different ballgame.

Also, which model transmission would this truck have in it?

I want to buy it, and he said he could proably hold on to it for me for a few years until i can afford it.

**CPNE** · 03-07-2004, 08:06 PM

Either 4:11 or 4:56. If it does not state an optional ratio in the glove box, it's 4:11

MrGoodpliers · 03-07-2004, 08:12 PM

My '70 GMC 3/4-dana 60 equipped truck lists 4.10 ratio on the glovebox. Hmmm... I thought they only listed the ratio if it was an option, and I thought that 4.10s would be standard.

lukecp · 03-07-2004, 08:13 PM

Thanks!
It didn't say anything about the rear on the glovebox sheet, only options listed on the sheet were the 4-speed tranny, special paint, and one other i forgot. Very plain truck, however it runs and drives great. Felt werid driving a truck with manual everything, i'm used to my '72 with PS/PB/AT. I kinda liked it though, lol.

stelth2002 · 03-07-2004, 08:15 PM

the famous sm465 is the most likely canidate for the tranny.

**CPNE** · 03-07-2004, 08:15 PM

Quote:

Originally posted by MrGoodpliers
My '70 GMC 3/4-dana 60 equipped truck lists 4.10 ratio on the glovebox. Hmmm... I thought they only listed the ratio if it was an option, and I thought that 4.10s would be standard.

I6 3/4 tons with certain trannys had 4:56 standard

Longhorn Man · 03-07-2004, 08:41 PM

The 1970 book doesn't have anything about gearing...it is kinda pathetic, however, the 71 shows 4.56 being standard gearing when the 307 engine is included, regardless on what tranny it has. Also, if the tranny is original, the SM465 is the only one that it could be.

lukecp · 03-07-2004, 08:45 PM

Well if i get it i may have to pop the cover and count the teeth then. I did just make a trip from Colorado to Missouri at 70mph the whole way, so i would think it would have to have 4.10 gears to do that, however a 327 has a reputation for being a tight-rapping little motor.....

Mike C · 03-07-2004, 09:30 PM

Originally the Dana axles have a sheet metal tag that has a part number and the gear ratio stamped on it. If some loser mechanic didn't leave it off after changing the gear oil, should still be there. Clean it off with carb cleaner and read gear ratio directly! Too cool.

Longhorn Man · 03-07-2004, 10:30 PM

a 4.56 geared truck should be able to hold 70 no problem...they have pretty tall tires on there.

lukecp · 03-07-2004, 10:42 PM

Once again, thanks for the help. I'll check the tag next time i look see the truck, if the tag is still there.
Another thing i was suprised at is that the frame on the C20 looks no thicker or no taller than the frame on my C10. Only difference i can see are stiffer springs, 8-lug wheel pattern, and the much beefier rear axle.......
Also, with 31" tires (which the truck has), online caluclators put it at 3460rpm@70 with 4.56 gears and 3111rpm@70 with 4.10 gears...

Longhorn Man · 03-07-2004, 11:01 PM

There you go...3500 RPM is nothing to a small block.

your frame is pretty much the same as a 1/2 ton unit. If you look at the trailing arms, they are a little beefier too.

**CPNE** · 03-08-2004, 09:00 AM

Quote:

Originally posted by Longhorn Man
The 1970 book doesn't have anything about gearing...it is kinda pathetic, however, the 71 shows 4.56 being standard gearing when the 307 engine is included, regardless on what tranny it has. Also, if the tranny is original, the SM465 is the only one that it could be.

Are your sure about this? My 71 GM resto pack shows 4:10 for all small block's and trannys combos except 307 V8 with powerglide or 350 or 307 V8 with a 3 speed column shift. Those got 4:57 gears. This is true for both Corp and Dana axles. (Dana's are actually 4.56

)

All 250's got 4:57 , the 292 got them only with the 3 speed.

The 402 got 3:54 dana only, except the longhorn with the leaf rear. It got 4:10 dana with the BB

Duffy · 03-08-2004, 01:18 PM

My GMC longhorn with original 402 has a Dana 60 leaf rear axle with 3:54 from the factory. This is behind a manual tranny of course (sm465).

Duffy

**CPNE** · 03-08-2004, 01:30 PM

Quote:

Originally posted by Duffy
My GMC longhorn with original 402 has a Dana 60 leaf rear axle with 3:54 from the factory. This is behind a manual tranny of course (sm465).

Duffy

My bad. I should have stated:

The 402 BB had only one option for rear end ratio which was a dana 60 with 3:54. Exception is the longhorn which allowed you to opt for a 4:11.

Your 3:54 rear is the standard RPO.

03-07-2004, 07:08 PM	#1
lukecp Formerly yellow72custom Join Date: May 2001 Location: Austin, TX Posts: 7,531	GMC C20 Questions My stepbrother has a '70 GMC 3/4 ton longbox he has been trying to sell me for quite some time now. I finally got to look at it and drive it today. It is an original Colorado truck, so the body is really solid. Needs 2 rockers and a small part of one cab corner, above the windshield, floors, and cab supports are rock solid. Unlike my '72, which needs alot of work in all the commonly rusted areas. Anyway, it has a coil spring Dana 60 in the rear. It is an orignal 307 truck, with a granny low 4-speed transmission. The original 307 is gone, replaced with a very well running 327 from a 1962 fullsize car (i verfied it with the engine suffix codes). So, what would the axle ratio in the rear be stock? I'm thinking 4.10, but the truck seemed really low geared so it may be lower. I have the axle ratios for a C10 figured out, but C20s are a different ballgame. Also, which model transmission would this truck have in it? I want to buy it, and he said he could proably hold on to it for me for a few years until i can afford it. __________________ '72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg

03-07-2004, 08:06 PM	#2
CPNE Resident Curmudgeon Join Date: Dec 1999 Location: NH Posts: 6,662	Either 4:11 or 4:56. If it does not state an optional ratio in the glove box, it's 4:11 __________________ Currently on or near the homestead: 67 Chevy SWB 2WD stepside 350/3 on tree (Pat's) 67 GMC SWB 2WD Fleet 402/auto (Brian's under construction) 67 Chevy 3/4 ton 2WD 402/auto (Business Hauler) 67 Chevy 1 ton dually 2WD 396/4 speed (Former business hauler, Needs TLC) 68 Chevy 1/2 ton Suburban 2WD 250 six/3 on tree (Brian's Needs TLC) 70 Chevy 3/4 ton 4WD 350/4 speed (Pat's - Disguised as a 68 GMC) 71 Chevy SWB stepside (Crushed by tree - parts donor) 72 Chevy 3/4 ton 4WD (Parts donor) 72 Chevy 3/4 ton 4WD Suburban (Parts Donor) 72 GMC 3/4 ton 4WD 292 six/4 speed (Mine - Disguised as a 67 GMC) 81 GMC 4WD Dually Dump Body 350/4 speed (Business Hauler) 82 Camaro Z/28 355/Super T-10 (Pat's toy) 93 Caprice 9C1 (Brian's Cop Car) 02 Toyota Camry (Reliable but a souless steel and plastic hulk) 2011 2SS RS Camaro M6 Factory Hurst Shifter Maybe I need to sell some of this crap Yet another Bozo with a sawz-all

03-07-2004, 08:12 PM	#3
MrGoodpliers Registered User Join Date: May 2002 Location: Gainesville, Georgia Posts: 704	My '70 GMC 3/4-dana 60 equipped truck lists 4.10 ratio on the glovebox. Hmmm... I thought they only listed the ratio if it was an option, and I thought that 4.10s would be standard. __________________ We made Oklahoma a little after 3, Randy & his brother Bob & my old GMC...

03-07-2004, 08:13 PM	#4
lukecp Formerly yellow72custom Join Date: May 2001 Location: Austin, TX Posts: 7,531	Thanks! It didn't say anything about the rear on the glovebox sheet, only options listed on the sheet were the 4-speed tranny, special paint, and one other i forgot. Very plain truck, however it runs and drives great. Felt werid driving a truck with manual everything, i'm used to my '72 with PS/PB/AT. I kinda liked it though, lol. __________________ '72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg

03-07-2004, 08:15 PM	#5
stelth2002 Young Gun wth Fast Inline Join Date: Jan 2002 Location: Idaho Posts: 1,475	the famous sm465 is the most likely canidate for the tranny. __________________ 1967 Chevrolet LWB with built 292! 415 ft. lbs of torque, 4 speed :O with 27% overdrive too, 3.73 Posi. Frame and body done... getting painted!!!

03-07-2004, 08:41 PM	#7
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	The 1970 book doesn't have anything about gearing...it is kinda pathetic, however, the 71 shows 4.56 being standard gearing when the 307 engine is included, regardless on what tranny it has. Also, if the tranny is original, the SM465 is the only one that it could be.

03-07-2004, 08:45 PM	#8
lukecp Formerly yellow72custom Join Date: May 2001 Location: Austin, TX Posts: 7,531	Well if i get it i may have to pop the cover and count the teeth then. I did just make a trip from Colorado to Missouri at 70mph the whole way, so i would think it would have to have 4.10 gears to do that, however a 327 has a reputation for being a tight-rapping little motor..... __________________ '72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg

03-07-2004, 09:30 PM	#9
Mike C Registered User Join Date: Dec 1999 Location: Austin, TX, USA Posts: 7,727	Originally the Dana axles have a sheet metal tag that has a part number and the gear ratio stamped on it. If some loser mechanic didn't leave it off after changing the gear oil, should still be there. Clean it off with carb cleaner and read gear ratio directly! Too cool. __________________ 44 Willys MB 52 M38A1 64 Corvette Coupe 68 Camaro 'vert LT1 & TH700 69 Z/28 355 12.6's @110 69 Chevy Short Step 4 1/2"/7" drop 72 Jimmy 4WD 4spd 4" & 35's 02 GMC 2500HD 4x4 Duramax

03-07-2004, 10:30 PM	#10
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	a 4.56 geared truck should be able to hold 70 no problem...they have pretty tall tires on there.

03-07-2004, 10:42 PM	#11
lukecp Formerly yellow72custom Join Date: May 2001 Location: Austin, TX Posts: 7,531	Once again, thanks for the help. I'll check the tag next time i look see the truck, if the tag is still there. Another thing i was suprised at is that the frame on the C20 looks no thicker or no taller than the frame on my C10. Only difference i can see are stiffer springs, 8-lug wheel pattern, and the much beefier rear axle....... Also, with 31" tires (which the truck has), online caluclators put it at 3460rpm@70 with 4.56 gears and 3111rpm@70 with 4.10 gears... __________________ '72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg

03-07-2004, 11:01 PM	#12
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	There you go...3500 RPM is nothing to a small block. your frame is pretty much the same as a 1/2 ton unit. If you look at the trailing arms, they are a little beefier too.

03-08-2004, 01:18 PM	#14
Duffy Registered User Join Date: Jan 2001 Location: Billings, MT Posts: 121	Dana behind a big block My GMC longhorn with original 402 has a Dana 60 leaf rear axle with 3:54 from the factory. This is behind a manual tranny of course (sm465). Duffy __________________ '72 GMC Sierra Grande Custom Camper



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