Replacing rear gears...go with 3.08 or 3.42? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Crazy-J · 03-14-2005, 04:20 PM

Hi,

I am going to replace my rear gears as they have chipped teeth. My '72 currently has 3.73's in it (did these come with 3.73's originally?). I am looking to try and improve fuel milage as well as give the engine some room to run...it accelerates nice, chirps second no problem, but completely runs out of steam on the freeway.

Would I be better going with a 3.08 or a 3.42? I am not sure how much power the engine is making (350, mild cam, performer rpm intake, performer 600cfm carb, velocity stack, long tube headers/x-pipe/2.5" dual exhaust...that's all I know of as it was built by the previous owner).

Running a 700R4 transmission.

Thanks

boraxman · 03-14-2005, 04:31 PM

Since you have a OD trans I would go w/ 3.42 Thats what I run w/ a Turbo 350 and im fine with them.

69 longhorn · 03-14-2005, 04:45 PM

With your 700r4,i would also say 3.42. I run factory 3.40's , with a .76 OD .....lets just say she cruises real easy on the freeway

69L

skokie · 03-14-2005, 04:52 PM

I have a 01 sierra ( i know not a 67-72)

that has the 4l60e with the 3.42 and its great. 75 mph is 2000 rpm with 29 inch tires.

Crazy-J · 03-14-2005, 04:55 PM

Awesome, thanks! I'll be going with the 3.42's then.

vfrdude · 03-14-2005, 05:07 PM

I'm runing 3.73's with a 700R4 and it runs great! I'm getting about 16MPG commuting back and forth to work. Lots of stop and go - it usually takes me about 1.5 hours to go 40 miles

Not sure what you mean by 'completely runs out of steam on the freeway.' My truck is probably turning 2100 RPM at 65.

I did have 3.08's and after installing the 700R4, those gears were way to high! So, if you were going to change 3.42's would be the way to go.

MandarinSixSeven · 03-14-2005, 05:09 PM

Definately 3.42's. I have 3.08's that I put in in place of my old 3.73's and it takes the fun away definately. 3.42's would be idea for what I want, a good blend between low end and economy. This spring the plan is to put the 3.73's back in, that's how annoying the 3.08's are.

vfrdude · 03-14-2005, 05:19 PM

I was on the fence about going with 3.42's or 3.73's... I finally figured I was only looking at an RPM difference of about 190 at 70MPH...2083 (3.42) and 2271(3.73) - So I went with the 3.73's.

69 longhorn · 03-14-2005, 05:37 PM

Well, here are the pros/cons......If you drive out on the open road a bunch, you may notice a little more fuel milage with the taller gears.....around town, the lower gears probably wont eat any more fuel......

L

Crazy-J · 03-14-2005, 05:40 PM

Yeah I don't know why, but the engine is just flat in 3rd gear on the freeway. I can get it to kick from 4th to 3rd but I don't really get going. I am thinking now that I am missing the kickdown because I can't get it to shift into passing gear. It just sort of lugs it's way along. So that's more of a problem with the transmission than the gears I am sure.

The 3.73's are fun but with the chipped teeth they do need to be changed. 3.42's for my daily driving will work out best I think. Gas mileage right now is 10 miles to the gallon..ouch

Longhorn Man · 03-14-2005, 08:59 PM

if your tranny isn't shifting right, you better look into that asap before it craps out on you. Have a tranny shop check the line pressure and adjust acordingly.

03-14-2005, 04:20 PM	#1
Crazy-J Registered User Join Date: Mar 2005 Location: Cool, CA Posts: 34	Replacing rear gears...go with 3.08 or 3.42? Hi, I am going to replace my rear gears as they have chipped teeth. My '72 currently has 3.73's in it (did these come with 3.73's originally?). I am looking to try and improve fuel milage as well as give the engine some room to run...it accelerates nice, chirps second no problem, but completely runs out of steam on the freeway. Would I be better going with a 3.08 or a 3.42? I am not sure how much power the engine is making (350, mild cam, performer rpm intake, performer 600cfm carb, velocity stack, long tube headers/x-pipe/2.5" dual exhaust...that's all I know of as it was built by the previous owner). Running a 700R4 transmission. Thanks

03-14-2005, 04:31 PM	#2
boraxman Keepin an eye out Join Date: Feb 2003 Location: So.Cal Posts: 3,920	Since you have a OD trans I would go w/ 3.42 Thats what I run w/ a Turbo 350 and im fine with them. __________________ 1970 Chevy C10 SWB 5.3 1996 Toyota Tacoma SR5 4x4 2007 Vespa GTS 250 Scooter ZIP 91351

03-14-2005, 04:45 PM	#3
69 longhorn Registered User Join Date: Aug 2001 Location: rock island,il,usa Posts: 5,382	With your 700r4,i would also say 3.42. I run factory 3.40's , with a .76 OD .....lets just say she cruises real easy on the freeway 69L __________________ http://community.webshots.com/user/hotrodhorn

03-14-2005, 04:52 PM	#4
skokie Registered User Join Date: Oct 2003 Location: San Jose Ca. Posts: 3,354	I have a 01 sierra ( i know not a 67-72) that has the 4l60e with the 3.42 and its great. 75 mph is 2000 rpm with 29 inch tires. __________________ RIP to my son Dylan. 5/17/2002 - 8/18/2018 If you want to know more about rare forms of pediatric cancer or if you want to help fund pediatric cancer research, please check out the website for the non-profit I started. https://slothforpedcancer.org 68 build thread http://67-72chevytrucks.com/vboard/s...d.php?t=223234

03-14-2005, 05:07 PM	#6
vfrdude Registered User Join Date: May 2002 Location: Oakley, Ca. USA Posts: 137	I'm runing 3.73's with a 700R4 and it runs great! I'm getting about 16MPG commuting back and forth to work. Lots of stop and go - it usually takes me about 1.5 hours to go 40 miles Not sure what you mean by 'completely runs out of steam on the freeway.' My truck is probably turning 2100 RPM at 65. I did have 3.08's and after installing the 700R4, those gears were way to high! So, if you were going to change 3.42's would be the way to go. __________________ Rudy Fierros '95 Honda VFR750F Used to have a '71 Chevy Stepside, Guards Red, Lowered 4" rear, 3" front, White Oak bed wood, 700R4 w/373 gears until some low life stole it!

03-14-2005, 04:55 PM	#5
Crazy-J Registered User Join Date: Mar 2005 Location: Cool, CA Posts: 34	Awesome, thanks! I'll be going with the 3.42's then.

03-14-2005, 05:09 PM	#7
MandarinSixSeven Seasonal Poster Join Date: Apr 2002 Location: Kelowna BC Canada Posts: 896	Definately 3.42's. I have 3.08's that I put in in place of my old 3.73's and it takes the fun away definately. 3.42's would be idea for what I want, a good blend between low end and economy. This spring the plan is to put the 3.73's back in, that's how annoying the 3.08's are. __________________ 67 GMC Short Step 350/4-Speed/3.08 Posi Front/Rear Sway Bars Kelowna BC, Canada Member of DTPW

03-14-2005, 05:19 PM	#8
vfrdude Registered User Join Date: May 2002 Location: Oakley, Ca. USA Posts: 137	I was on the fence about going with 3.42's or 3.73's... I finally figured I was only looking at an RPM difference of about 190 at 70MPH...2083 (3.42) and 2271(3.73) - So I went with the 3.73's. __________________ Rudy Fierros '95 Honda VFR750F Used to have a '71 Chevy Stepside, Guards Red, Lowered 4" rear, 3" front, White Oak bed wood, 700R4 w/373 gears until some low life stole it!

03-14-2005, 05:37 PM	#9
69 longhorn Registered User Join Date: Aug 2001 Location: rock island,il,usa Posts: 5,382	Well, here are the pros/cons......If you drive out on the open road a bunch, you may notice a little more fuel milage with the taller gears.....around town, the lower gears probably wont eat any more fuel...... L __________________ http://community.webshots.com/user/hotrodhorn

03-14-2005, 05:40 PM	#10
Crazy-J Registered User Join Date: Mar 2005 Location: Cool, CA Posts: 34	Yeah I don't know why, but the engine is just flat in 3rd gear on the freeway. I can get it to kick from 4th to 3rd but I don't really get going. I am thinking now that I am missing the kickdown because I can't get it to shift into passing gear. It just sort of lugs it's way along. So that's more of a problem with the transmission than the gears I am sure. The 3.73's are fun but with the chipped teeth they do need to be changed. 3.42's for my daily driving will work out best I think. Gas mileage right now is 10 miles to the gallon..ouch

03-14-2005, 08:59 PM	#11
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	if your tranny isn't shifting right, you better look into that asap before it craps out on you. Have a tranny shop check the line pressure and adjust acordingly.



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