Rear ratio question. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

krue · 08-13-2017, 01:43 PM

I'm putting a 6.0l/4l80e combo in my 70. I currently have a 3.07 ratio rear end and wonder how well this will work with the engine/tranny combo. When I was planning a 5.3l or 4.8l conversion I planned on going with a 3.42 ratio rear end. I wonder if the added torque will make the 3.07 work. Plan on daily driver with occasional towing duty (pulling my tractor or vehicles around).

**nlped** · 08-13-2017, 05:15 PM

How tall of a tire will you be running? If it isn't really short, that 3.07 gear is going to cause the truck to go in & out of overdrive at highway speed.... unless you're going to be driving 100+ mph

Eddie H. · 08-13-2017, 05:41 PM

I went with a 3:73 on the last overdrive conversion I did. It was geared way too high with the 3:07.

Shinysideup · 08-13-2017, 06:05 PM

You'll want the 3.70, luckily that came stock so you can find a complete rear to swap.

Daves70 · 08-13-2017, 08:14 PM

3.73 seems to be the sweet spot gear with an overdrive. I have a 3.42 and it's still a little too tall for my combo.

ozarks hunter · 08-13-2017, 08:26 PM

I'm doing the same swap in my 72 but with a 410 rear gear. Hopefully I'll be able to let you know how that combo works in a few months.

**LockDoc** · 08-13-2017, 08:54 PM

-
I also vote for the 3.73. The 3.07 will be too steep. It may cause your trans to overheat.

LockDoc

**Wrenchbender Ret** · 08-13-2017, 09:24 PM

I had a 70 with 350 engine, 3:08 rear gears & 700R4. It was a pleasure to drive. It would pull long hills on the Interstate with A/C on & never downshift. The engine was quite & the MPG good. If you were pulling a load you just left it in direct drive & it still worked good. The best of 2 worlds.

MattPSU · 08-14-2017, 08:41 AM

I have a 5.3 with a BTR stage 3 cam, 4l60e and my truck originally had 3.07 gears. With the 3.07, I felt like it was searching for gears all the time and the trans ran hot unless I was cruising on the highway at 70mph. At highway speeds it was great, but not a great combo for around town. I actually considered going back to a stock cam because I felt like it wasn't ever in the sweet spot RPM-wise for the cam.

I just put in 3.42's mostly because I picked up a posi unit cheap and that was the lowest ratio it would accept. I have just over 100 miles on it so far, but I'm liking it a lot more. It seems to have woken the truck up considerably. It seems quicker at the light, it's not searching for gears, and my transmission seems to be running cooler.

My researched found that 3.73 seemed to be the ideal LS swap gear, and that's what I would have gone with had I not picked up the posi so cheap.

08-13-2017, 01:43 PM	#1
krue Designated A-hole! Join Date: Mar 2003 Location: South Mississippi Posts: 36,450	Rear ratio question. I'm putting a 6.0l/4l80e combo in my 70. I currently have a 3.07 ratio rear end and wonder how well this will work with the engine/tranny combo. When I was planning a 5.3l or 4.8l conversion I planned on going with a 3.42 ratio rear end. I wonder if the added torque will make the 3.07 work. Plan on daily driver with occasional towing duty (pulling my tractor or vehicles around). __________________ "If it ain't broke, fix it 'til it is!" Being stupid ain't illegal. We're Still Debt Free Except for the House!!! www.daveramsey.com 70 GMC SWB Stepside project "Green Booger" soon to be 6.0l/4l80e 93 S-10 "Poppaw's Truck"

08-13-2017, 05:15 PM	#2
nlped Senior Member Join Date: Jul 2009 Location: Hayden, Al. Posts: 4,147	Re: Rear ratio question. How tall of a tire will you be running? If it isn't really short, that 3.07 gear is going to cause the truck to go in & out of overdrive at highway speed.... unless you're going to be driving 100+ mph __________________ Carter 1968 Chevy SWB Fleet... Walt 1963-1/2 Ford Falcon Futura...Martha

08-13-2017, 05:41 PM	#3
Eddie H. Registered User Join Date: Mar 2001 Location: Paradise TX USA Posts: 1,200	Re: Rear ratio question. I went with a 3:73 on the last overdrive conversion I did. It was geared way too high with the 3:07. __________________ "Negative people always seem to have a problem for every solution"

08-13-2017, 08:14 PM	#5
Daves70 Registered User Join Date: Jul 2011 Location: Mt Brydges Ontario Posts: 180	Re: Rear ratio question. 3.73 seems to be the sweet spot gear with an overdrive. I have a 3.42 and it's still a little too tall for my combo.

08-13-2017, 08:26 PM	#6
ozarks hunter Registered User Join Date: May 2013 Location: Caulfield, Mo. Posts: 35	Re: Rear ratio question. I'm doing the same swap in my 72 but with a 410 rear gear. Hopefully I'll be able to let you know how that combo works in a few months.

08-13-2017, 08:54 PM	#7
LockDoc The Older Generation Join Date: Aug 2006 Location: Montezuma, Iowa Posts: 25,331	Re: Rear ratio question. - I also vote for the 3.73. The 3.07 will be too steep. It may cause your trans to overheat. LockDoc __________________ Leon Locksmith, Specializing In Antique Trucks, Automobiles, & Motorcycles (My Dually Pickup Project Thread) http://67-72chevytrucks.com/vboard/s...d.php?t=829820 -

08-13-2017, 09:24 PM	#8
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,190	Re: Rear ratio question. I had a 70 with 350 engine, 3:08 rear gears & 700R4. It was a pleasure to drive. It would pull long hills on the Interstate with A/C on & never downshift. The engine was quite & the MPG good. If you were pulling a load you just left it in direct drive & it still worked good. The best of 2 worlds.

08-14-2017, 08:41 AM	#9
MattPSU Registered User Join Date: Nov 2011 Location: Lyman, SC Posts: 734	Re: Rear ratio question. I have a 5.3 with a BTR stage 3 cam, 4l60e and my truck originally had 3.07 gears. With the 3.07, I felt like it was searching for gears all the time and the trans ran hot unless I was cruising on the highway at 70mph. At highway speeds it was great, but not a great combo for around town. I actually considered going back to a stock cam because I felt like it wasn't ever in the sweet spot RPM-wise for the cam. I just put in 3.42's mostly because I picked up a posi unit cheap and that was the lowest ratio it would accept. I have just over 100 miles on it so far, but I'm liking it a lot more. It seems to have woken the truck up considerably. It seems quicker at the light, it's not searching for gears, and my transmission seems to be running cooler. My researched found that 3.73 seemed to be the ideal LS swap gear, and that's what I would have gone with had I not picked up the posi so cheap. __________________ Matt Criswell - Lyman, SC 1972 Chevy Cheyenne SWB - Build: Porterbuilt State 1 Front & Rear, Bolt in Notch, McGaughys 2.5 Drop Spindles, Boyd Welding Fuel Tank UPSTATE SC GM TRUCK CLUB: FACEBOOK WEBSITE



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