Rear end gears - The 1947 - Present Chevrolet & GMC Truck Message Board Network

del4fun · 11-10-2021, 12:55 PM

What number rear end gears do you use with a 700 R tranny on a 72 C10
Also I have 20 inch tires and wheels

**jocko** · 11-10-2021, 01:03 PM

Need your outer tire diameter to give best answer. 3.73 is usually a good ratio with a 709R4, but if you have a much shorter or taller than stock tire, the answer will be different.

LONGHAIR · 11-10-2021, 01:13 PM

Tire diameter is far more useful than wheel, which doesn't really mean much.
Also driving conditions/requirements are needed too.
Towing versus highway driving, etc. also, believe it or not, carburetor or fuel injection?
Just to throw MOTR out there though, probably around 3.73, that might vary a bit based on which axle you have too. Some axles have limited ratios available for them.
If you tow more often (or heavy) go lower 4.10, if you drive fast on the highway more, go higher 3.50

RustyBucket · 11-10-2021, 04:43 PM

Goodrich says my 275/60/15 T/A Radial is 28" tall. My '71 GMC step-side has a 3.08 rear, 700r4, and a mild 350 engine. First gear in the 700r4 is a low 3.06 ratio, so the truck jumps off from a stop good. Under light throttle it shifts, 17-20mph. Pretty big gap in ratio first to second gear(1.63) but it pulls it ok and keeps up with traffic good. Shifts itself under light throttle again around 30mph. and in 3rd gear ratio is 1.1, just like top gear in a Turbo 350. A lot of the time driving around town I bump the shift lever up 1 notch so it doesn't go into overdrive. If I don't do that it will shift itself under light throttle into O.D. around 50mph. I don't have a tach. in the truck but engine just feels "happier" turning a few more R.P.M.s. Mileage isn't great. I do baby the transmission a little 'cause I don't think it will stand much abuse..... I run the pee out of my Blazer anytime nobody's lookin'.
Looks like with 3.73's that low first gear would be mighty short.

del4fun · 11-10-2021, 07:41 PM

Quote:

Originally Posted by LONGHAIR

Tire diameter is far more useful than wheel, which doesn't really mean much.
Also driving conditions/requirements are needed too.
Towing versus highway driving, etc. also, believe it or not, carburetor or fuel injection?
Just to throw MOTR out there though, probably around 3.73, that might vary a bit based on which axle you have too. Some axles have limited ratios available for them.
If you tow more often (or heavy) go lower 4.10, if you drive fast on the highway more, go higher 3.50

Great info

del4fun · 11-10-2021, 07:45 PM

Quote:

Originally Posted by RustyBucket

Goodrich says my 275/60/15 T/A Radial is 28" tall. My '71 GMC step-side has a 3.08 rear, 700r4, and a mild 350 engine. First gear in the 700r4 is a low 3.06 ratio, so the truck jumps off from a stop good. Under light throttle it shifts, 17-20mph. Pretty big gap in ratio first to second gear(1.63) but it pulls it ok and keeps up with traffic good. Shifts itself under light throttle again around 30mph. and in 3rd gear ratio is 1.1, just like top gear in a Turbo 350. A lot of the time driving around town I bump the shift lever up 1 notch so it doesn't go into overdrive. If I don't do that it will shift itself under light throttle into O.D. around 50mph. I don't have a tach. in the truck but engine just feels "happier" turning a few more R.P.M.s. Mileage isn't great. I do baby the transmission a little 'cause I don't think it will stand much abuse..... I run the pee out of my Blazer anytime nobody's lookin'.
Looks like with 3.73's that low first gear would be mighty short.

4.10 it is , thanks

**jocko** · 11-10-2021, 07:56 PM

so, what is your tire diameter?

Accelo · 11-10-2021, 10:29 PM

I have a stock 2000 Silverado with the 5.3L motor and the 4L60E transmission.
The factory differential ratio is 3:70’s. Straight highway mileage is 19. l also have a set of 20” wheels on my C10. The height of the Sliverado's stock 16” tires is almost identical to the C10's 20” wheel and tire combination. I don’t know anyone towing with a 67-72 C10. I’m sure people tow I just don’t see them.

With the lower 4L60 transmission’s first gear it will offset some of the differential gear differences. THM 350 1st gear is 2.52. The 4L60 1st gear is 3.06. The total 1st gear ratio is actually lower using 4L60 and the 3:70 ratio when compared to a stock THM350 and the 4:11.

My recommendation is to use the 3:70. GM knows what they are doing. Contrary to what every magazine I read when I was young.

BCOWANWHEELS · 11-11-2021, 04:35 AM

Go to randys ring and pinion web site they have a gear ratio / rpm calculator there i,d say 3.73 up to 4.11

LONGHAIR · 11-11-2021, 07:02 PM

Quote:

Originally Posted by Accelo

My recommendation is to use the 3:70. GM knows what they are doing. Contrary to what every magazine I read when I was young.

Please don't fall into that trap. They have done some seriously dumb stuff over the years.
Even in just the gear ratio thing, they had at least one flaming doosey.
In the early 80s, in a 4 wheel drive truck, with a 700R4....2.56 gears? Ahh, no.

71meangreenc10 · 11-11-2021, 08:17 PM

The majority of GM 1/2 ton trucks with OD will have 3.42 rear axle gear ratio.

I put it in my conversions and can find parts for the speedo if I use mechanical speedo's.

Smitty

**davischevy** · 11-11-2021, 09:55 PM

My Longhorn has 31" tires with 3.73 tires, and works great with the big block.

11-10-2021, 12:55 PM	#1
del4fun Registered User Join Date: Nov 2011 Location: Vacaville, California Posts: 215	Rear end gears What number rear end gears do you use with a 700 R tranny on a 72 C10 Also I have 20 inch tires and wheels __________________ US ARMY RET, SUPPORT THE TROOPS 1972 Custom/10 Short Fleet 4.5/ 5 Drop Dogg Pound Customs

11-10-2021, 01:03 PM	#2
jocko Senior Member Join Date: Oct 2009 Location: Godley, TX Posts: 17,934	Re: Rear end gears Need your outer tire diameter to give best answer. 3.73 is usually a good ratio with a 709R4, but if you have a much shorter or taller than stock tire, the answer will be different.

11-10-2021, 04:43 PM	#4
RustyBucket Registered User Join Date: Jul 2009 Location: Space Coast, Fl. Posts: 1,050	Re: Rear end gears Goodrich says my 275/60/15 T/A Radial is 28" tall. My '71 GMC step-side has a 3.08 rear, 700r4, and a mild 350 engine. First gear in the 700r4 is a low 3.06 ratio, so the truck jumps off from a stop good. Under light throttle it shifts, 17-20mph. Pretty big gap in ratio first to second gear(1.63) but it pulls it ok and keeps up with traffic good. Shifts itself under light throttle again around 30mph. and in 3rd gear ratio is 1.1, just like top gear in a Turbo 350. A lot of the time driving around town I bump the shift lever up 1 notch so it doesn't go into overdrive. If I don't do that it will shift itself under light throttle into O.D. around 50mph. I don't have a tach. in the truck but engine just feels "happier" turning a few more R.P.M.s. Mileage isn't great. I do baby the transmission a little 'cause I don't think it will stand much abuse..... I run the pee out of my Blazer anytime nobody's lookin'. Looks like with 3.73's that low first gear would be mighty short. __________________ Dad always said, "Son, WISH IN ONE HAND, and ......." -------------------------------------- -------------------------------------- Current toy trucks: '71 GMC 1500 SUPER CUSTOM short/step, orig. dk. blue, 350/700r, ps,pb, A/C '72 Blazer 2wd, ochre & white, 454, tremecTKO 500 5spd. ps, pb, A/C, tilt Last 10yrs of hobby vehicles, had a FEW more in the 50yrs. before these: '66 Plymouth Belvedere City of Miami cop car clone. '70 Nova 406"sb, 13.5-1, solid roller, Brodix, p.glide/t.brake, back halved, 9" Ford, spool 4.88, cage,ladder bar/coilover, 10.5 tire....... SOLD!! '67 C10 short/step side mount spare -- SOLD!! '72 Jimmy 2wd, 350/350, ps, pb, fun driver, lots of bondo & a shiny red paint job..... SOLD!! '69 Nova 350, 4spd, A/C, ps, p.b, ...SOLD!!

11-10-2021, 07:56 PM	#7
jocko Senior Member Join Date: Oct 2009 Location: Godley, TX Posts: 17,934	Re: Rear end gears so, what is your tire diameter?

11-10-2021, 10:29 PM	#8
Accelo Registered User Join Date: Mar 2015 Location: washington Posts: 2,163	Re: Rear end gears I have a stock 2000 Silverado with the 5.3L motor and the 4L60E transmission. The factory differential ratio is 3:70’s. Straight highway mileage is 19. l also have a set of 20” wheels on my C10. The height of the Sliverado's stock 16” tires is almost identical to the C10's 20” wheel and tire combination. I don’t know anyone towing with a 67-72 C10. I’m sure people tow I just don’t see them. With the lower 4L60 transmission’s first gear it will offset some of the differential gear differences. THM 350 1st gear is 2.52. The 4L60 1st gear is 3.06. The total 1st gear ratio is actually lower using 4L60 and the 3:70 ratio when compared to a stock THM350 and the 4:11. My recommendation is to use the 3:70. GM knows what they are doing. Contrary to what every magazine I read when I was young. Last edited by Accelo; 11-11-2021 at 02:11 PM.

11-11-2021, 04:35 AM	#9
BCOWANWHEELS Registered User Join Date: Nov 2006 Location: KINGSPORT,TN. Posts: 3,035	Re: Rear end gears Go to randys ring and pinion web site they have a gear ratio / rpm calculator there i,d say 3.73 up to 4.11 __________________ I BELIEVE IN JOHN 3:16

11-11-2021, 08:17 PM	#11
71meangreenc10 Its a Truck Thing...... Join Date: Dec 2004 Location: Sierra Vista, Arizona Posts: 3,158	Re: Rear end gears The majority of GM 1/2 ton trucks with OD will have 3.42 rear axle gear ratio. I put it in my conversions and can find parts for the speedo if I use mechanical speedo's. Smitty __________________ 71 C10 283/3SPD Full Resto 71 GMC 1500 Sierra Grande http://67-72chevytrucks.com/vboard/s...d.php?t=518599 70 C10 Suburban Former military GSA truck. 72 Chevy Blazer 4X4, Sloppy Jo, Mountain Climber. Wife says no more trucks. http://67-72chevytrucks.com/vboard/s...d.php?t=707378 72 GMC 1 Ton Motor Home, wife said no more trucks until she saw this one. Gen 3 6.0/4L80E 4.10 http://67-72chevytrucks.com/vboard/s...d.php?t=761110 68 GMC Suburban was 3/4 ton, now 1/2.Wife shook her head 71 C30 Wrecker 71 C20 Scott-Bilt As weird as it gets..BB Cheyenne AC Truck 68 GMC Long Stepside. They keep following me home 71 C20 Longhorn/Ole Yellor 69 C30 Former Motor Home, Flat Bed time

11-11-2021, 09:55 PM	#12
davischevy Senior Member Join Date: Sep 2008 Location: Harrison, Arkansas Posts: 9,790	Re: Rear end gears My Longhorn has 31" tires with 3.73 tires, and works great with the big block. __________________ other Larry Build thread, Arkansas K10 https://67-72chevytrucks.com/vboard/...=755797&page=5 The ability to speak several languages is an asset, but the ability to keep your mouth shut in any language is priceless.



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