rear end ratio opinions - The 1947 - Present Chevrolet & GMC Truck Message Board Network

steeeeeve · 07-19-2020, 09:12 AM

I've got '68 C10
new 350 V8 -Nothing special 250HP approx
Turbo 400
Stock 15" wheels
currently 4.10 rear end

Not looking to do burnouts or win any races off the line. Just want to be able to easily cruise at 65-70 without winding out the engine.

I'm thinking 3.23

What do you think?

thanks,
steve

yuccales · 07-19-2020, 10:26 AM

The 3.08's in my 70 putts along just fine in town and on the highway. I don't use it to tow anything.

**LockDoc** · 07-19-2020, 10:58 AM

Quote:

Originally Posted by steeeeeve

I've got '68 C10
new 350 V8 -Nothing special 250HP approx
Turbo 400
Stock 15" wheels
currently 4.10 rear end

Not looking to do burnouts or win any races off the line. Just want to be able to easily cruise at 65-70 without winding out the engine.

I'm thinking 3.23

What do you think?

thanks,
steve

Not sure if you were actually thinking 3.73 or not, I don't think there is a 3.23 for these trucks. There was a 3.36 but I'm not sure what it was used in.

If you were thinking 3.73 I don't think it will help you that much. The 3.07 would definitely give you a higher top end but I don't know how it would do on the bottom end with the lower HP 350.

LockDoc

RustyBucket · 07-19-2020, 12:03 PM

Always heard about OPINIONS being like "REAR ENDS"....... and truly, most everybody's got one of each.

I have 3.07's in both of my trucks. The GMC has a mild 350/700r and it'll cruise 60-70mph comfortably in 3rd gear (1-1 ratio). Not lazy off the line either. 700r does have a lower 1st, gear ratio than your 400 trans. You might find a
complete rear end on CL or Marketplace.. most I see are around here are 3.07's. It'd be cheaper than buying a gears & having them fitted, unless you can set one up.

gmc684x4 · 07-19-2020, 12:09 PM

you can go for your carrier is 3.73 any gearing smaller needs a new center section
Many of our trucks are 3.08 or 3.73 in 1/2 tons

Here is a quick rpm calculator

https://www.differentials.com/techni...io-calculator/

gmc684x4 · 07-19-2020, 12:12 PM

I would look at 700r4 or a built 200r4 and add overdrive with out the need for computer control

https://www.hotrod.com/articles/ccrp...mission-build/

SWEET7T · 07-19-2020, 12:48 PM

3.08 gear set. Turns 2500rpm at 65-70. I have a 307ci with a 2-barrel. High altitude and steep hills will slow it down, but I've managed to stay above 60 on the climbs on the interstates.

**jocko** · 07-19-2020, 04:12 PM

I'd retain the rear as is because while 3.73 will be nicer on the highway - it's not as nice as a 4.10 with 700R4 O/D, which makes your 4.10s a 2.87 at highway speeds - and retains the exact same current acceleration performance in 1st-3rd that you have today. I'd get built 700R4 though, not a junkyard swap, because 4.10s can be kinda rough on it if you do the occasional burnout...

Minkota · 07-20-2020, 09:15 AM

Quote:

Originally Posted by steeeeeve

I've got '68 C10
new 350 V8 -Nothing special 250HP approx
Turbo 400
Stock 15" wheels
currently 4.10 rear end

You did not mention the Tire height, that can make a difference as well.

tutone · 07-20-2020, 09:22 AM

I had almost exactly the same set up in my last 72. 3.07 or 3.08, whatever you want to call them, are all you will need with that combo. I ran 2.35/75/15s on mine and got 15 mpg if I behaved. Even driving it hard, it wasn't all that bad.

68Stepbed · 07-20-2020, 09:33 AM

3.08 will be a good gear if you just plan to cruise around and be able to run at highway speeds. If you want a tad more punch off the line and still be able to run highway speeds, 3.42 is a great gear.

The only aftermarket gear options for the stock 12 bolt truck rearend are 3.08, 3.42, 3.73, 4.11, 4.56, 4.88, and some offer a 5.13.

Keep in mind, to change to a 3.42 or 3.08 from the current 4.11 will require a new carrier. 3.73 and lower gears are a 4-series carrier. 3.42 and 3.08 use a 3-series carrier. The difference is ring gear thickness. This may not help your case, but gearsets are available to put a 4-series gear on a 3-series carrier, but not the other way around.

TwoFiftyShifter · 07-21-2020, 08:27 PM

If 70mph cruise is your priority, the 3.07 or an OD trans. Even with 3.42s, you would be at 2800 at 70. Not exactly a mellow cruise. With a 3.07 gear swap, you won't be towing anything - which probably isn't happening anyway.

**HO455** · 07-21-2020, 10:32 PM

Shining a little science on the discussion.

http://www.alloyusa.com/gear-ratio-calculator/

07-19-2020, 09:12 AM	#1
steeeeeve Registered User Join Date: Nov 2018 Location: austin Texas Posts: 37	rear end ratio opinions I've got '68 C10 new 350 V8 -Nothing special 250HP approx Turbo 400 Stock 15" wheels currently 4.10 rear end Not looking to do burnouts or win any races off the line. Just want to be able to easily cruise at 65-70 without winding out the engine. I'm thinking 3.23 What do you think? thanks, steve

07-19-2020, 10:26 AM	#2
yuccales Registered User Join Date: May 2011 Location: Northern Calif. Posts: 3,816	Re: rear end ratio opinions The 3.08's in my 70 putts along just fine in town and on the highway. I don't use it to tow anything.

07-19-2020, 12:03 PM	#4
RustyBucket Registered User Join Date: Jul 2009 Location: Space Coast, Fl. Posts: 1,055	Re: rear end ratio opinions Always heard about OPINIONS being like "REAR ENDS"....... and truly, most everybody's got one of each. I have 3.07's in both of my trucks. The GMC has a mild 350/700r and it'll cruise 60-70mph comfortably in 3rd gear (1-1 ratio). Not lazy off the line either. 700r does have a lower 1st, gear ratio than your 400 trans. You might find a complete rear end on CL or Marketplace.. most I see are around here are 3.07's. It'd be cheaper than buying a gears & having them fitted, unless you can set one up. __________________ 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!!

07-19-2020, 12:09 PM	#5
gmc684x4 Registered User Join Date: Jul 2012 Location: Whitehorse yukon Posts: 1,218	Re: rear end ratio opinions you can go for your carrier is 3.73 any gearing smaller needs a new center section Many of our trucks are 3.08 or 3.73 in 1/2 tons Here is a quick rpm calculator https://www.differentials.com/techni...io-calculator/

07-19-2020, 12:12 PM	#6
gmc684x4 Registered User Join Date: Jul 2012 Location: Whitehorse yukon Posts: 1,218	Re: rear end ratio opinions I would look at 700r4 or a built 200r4 and add overdrive with out the need for computer control https://www.hotrod.com/articles/ccrp...mission-build/

07-19-2020, 12:48 PM	#7
SWEET7T Try spinnin 4 rear tars Join Date: Sep 2003 Location: Salt Lake City, Utah Posts: 757	Re: rear end ratio opinions 3.08 gear set. Turns 2500rpm at 65-70. I have a 307ci with a 2-barrel. High altitude and steep hills will slow it down, but I've managed to stay above 60 on the climbs on the interstates. __________________ SWEET7T 1970 C10, 2wd, LB, 307/TH350/3.08. PS, PB

07-19-2020, 04:12 PM	#8
jocko Senior Member Join Date: Oct 2009 Location: Godley, TX Posts: 17,994	Re: rear end ratio opinions I'd retain the rear as is because while 3.73 will be nicer on the highway - it's not as nice as a 4.10 with 700R4 O/D, which makes your 4.10s a 2.87 at highway speeds - and retains the exact same current acceleration performance in 1st-3rd that you have today. I'd get built 700R4 though, not a junkyard swap, because 4.10s can be kinda rough on it if you do the occasional burnout...

07-20-2020, 09:22 AM	#10
tutone Registered User Join Date: Mar 2016 Location: hickory, ky Posts: 1,342	Re: rear end ratio opinions I had almost exactly the same set up in my last 72. 3.07 or 3.08, whatever you want to call them, are all you will need with that combo. I ran 2.35/75/15s on mine and got 15 mpg if I behaved. Even driving it hard, it wasn't all that bad. __________________ :My build thread http://67-72chevytrucks.com/vboard/s...d.php?t=807535 1972 K10 swb/1968 Camaro 350/4spd 1955 210 2 dr sedan/1998 RCSB Z71/1977 Jeep CJ5

07-20-2020, 09:33 AM	#11
68Stepbed Registered User Join Date: Mar 2005 Location: 10-Uh-See Posts: 5,609	Re: rear end ratio opinions 3.08 will be a good gear if you just plan to cruise around and be able to run at highway speeds. If you want a tad more punch off the line and still be able to run highway speeds, 3.42 is a great gear. The only aftermarket gear options for the stock 12 bolt truck rearend are 3.08, 3.42, 3.73, 4.11, 4.56, 4.88, and some offer a 5.13. Keep in mind, to change to a 3.42 or 3.08 from the current 4.11 will require a new carrier. 3.73 and lower gears are a 4-series carrier. 3.42 and 3.08 use a 3-series carrier. The difference is ring gear thickness. This may not help your case, but gearsets are available to put a 4-series gear on a 3-series carrier, but not the other way around. __________________ Matt 68 C10 stepside, LS1/700R4, TCI Engineering suspension system

07-21-2020, 08:27 PM	#12
TwoFiftyShifter Registered User Join Date: Nov 2008 Location: Gwinnett GA Posts: 1,803	Re: rear end ratio opinions If 70mph cruise is your priority, the 3.07 or an OD trans. Even with 3.42s, you would be at 2800 at 70. Not exactly a mellow cruise. With a 3.07 gear swap, you won't be towing anything - which probably isn't happening anyway. __________________ 1982 K10 SWB 1987 V20 - Sold - Doh!

07-21-2020, 10:32 PM	#13
HO455 Post Whore Join Date: Feb 2016 Location: Portland Oregon Posts: 11,605	Re: rear end ratio opinions Shining a little science on the discussion. http://www.alloyusa.com/gear-ratio-calculator/ __________________ Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. RIP Bob Parks. 1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377



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