73 leaf rear in 72 leaf truck????? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**JQ-72** · 08-14-2009, 01:06 PM

Will a leaf spring rear out of a 73-80 model truck (12 bolt) bolt directly into a 72 GMC with leaf springs? I found one with 2.73 gear ratio and wanted to squeeze out an extra mile or 2 out of a gallon of gas - or is it even worth it?? I have 3.08's now. Thanks for any help.

Joshua

Zachary Watts · 08-14-2009, 01:11 PM

Yes it will bolt in but not directly you will have to relocate one of the shocks hangers to the front of the rearend.

Arizona oldguy · 08-14-2009, 01:31 PM

I did this exact swap, I found the rear end still attached to an abandoned 73 long bed, I wanted a lower ratio, and it is a 3:42 I cut the shock mount off of the rear and re-welded it to the front. I also found out that the spacing of the shock mounts are different they are closer together on the 72 I had to use a grade 8 bolt and make it like a shock mount stud, if I had it to do over again I would have cut them both off and re-welded them in the correct place.

capev86 · 08-14-2009, 01:35 PM

3.08 is kinda lame unless you have a huge motor (like a diesel) and only do light duty towing. going from a 3.08 to 3.73 in my dad's 87 burb 2wd really woke up that 350. 2.73's are good for economy cars because they weigh a lot less and have much shorter tires. on a truck, the larger tires act like an OD. thats why you want a decent gear. 2.73 sounds like slow as molasses to me unless you have a mean engine under the hood. good luck!

hint: 3.73 x .7 OD = 2.61 compare 2.61 to straight up 2.73.....more power and lower rpm's.

69GMCLonghorn · 08-14-2009, 04:32 PM

Yeah, what you have to realize is sometimes you need MORE gears, not just higher gears (numerically lower). As stated above, if you keep some short tires on it, you'll have a lighter rear end and less rolling weight/resistance.

Now, in my opinion, lightening up the gears and tires is kind of like putting perfume on a pig; it probably won't net much of a difference. The gear ratio? probably... give it a shot. If it's too high a gear, you know you can always go with lower (numerically higher) gears. I had 2.73's in a '71 Chevelle with 235-60-15 tires on it. I was taching about 2400 at 70 MPH, which was just about right. I used to get between 17 & 19 MPG out of it...

68gmsee · 08-14-2009, 05:25 PM

As mentioned that 2.73 may be sorta weak if you intend to do any towing. If you just toodle around mostly on the highway, then it may buy you something but it's not than much of a difference from the 3.08.

I went from a 3.54 or something like that to a 3.08 on my 69 and mpg didn't improve much if any. Also, I've towed a 16ft trailer with a tractor and bush hog and didn't have any problems with the 350/350 it's got in it.

badpeanut · 08-14-2009, 09:00 PM

Quote:

Originally Posted by JQ-72

Will a leaf spring rear out of a 73-80 model truck (12 bolt) bolt directly into a 72 GMC with leaf springs? I found one with 2.73 gear ratio and wanted to squeeze out an extra mile or 2 out of a gallon of gas - or is it even worth it?? I have 3.08's now. Thanks for any help.

Joshua

My '72 GMC had a 2.73 Gov-Lok '73-up rearend when I bought it:

the '73-up spring pads were torched narrower to adjust from 2.5" wide springs to 2.0" wide springs
the '73-up rearend had no bump stop provisions so the PO didn't bother
both shock mounts were relocated to different spots on the front of the '73-up rearend
the '73-up rearend had the wider brake shoes

I swapped to a '70 3.08 posi and am MUCH happier. The '73-up installation was a hack job, and the ratio made the truck slugish (350/TH350). The only thing I liked about the '73-rearend was the brakes (I saved everything from the backing plates out and may swap them on someday). Gas mileage is about the same (I think slipage became an issue with the 2.73s).

PanelDeland · 08-14-2009, 09:12 PM

Keep in mind that a 2:73 may lower your RPM's to the point that the engine is out of it's "sweet spot"RPM wise(torque and HP).This may actually worsen your fuel milage.It's could almost be like lugging your engine all the time.

**JQ-72** · 08-15-2009, 07:26 AM

Thanks for all your input. I think you've talked me out of it!!

JQ

**Stocker** · 08-15-2009, 10:16 AM

Quote:

Originally Posted by PanelDeland

Keep in mind that a 2:73 may lower your RPM's to the point that the engine is out of it's "sweet spot"RPM wise(torque and HP).This may actually worsen your fuel milage.It's could almost be like lugging your engine all the time.

Very true, and if you're still interested in a bit of experimentation, swap out the rear tires for taller ones. Cheap & easy to do, especially compared to swapping out the rear axle & springs, and it will effectively raise the gear ratio.

glock35ipsc · 08-23-2009, 02:45 PM

So GM got it wrong when they used the 350/TH350/2.73 combo for decades?

Longhorn Man · 08-23-2009, 06:57 PM

MPG will probably drop. many people on here has done the 700R4/3.07 gear and reported worse MPG.
GM got a lot of stuff wrong, and the real tall gear in a truck was one of them.

**special-K** · 08-24-2009, 08:24 AM

The GM trucks with 2.73s actually drove ok.I`d never run them,but I know of plenty of trucks that satisfied their owners with that combo.

glock35ipsc · 08-24-2009, 09:09 AM

Quote:

Originally Posted by special-K

The GM trucks with 2.73s actually drove ok.I`d never run them,but I know of plenty of trucks that satisfied their owners with that combo.

Although the entire combo is coming out in a few weeks, I really like driving mine with the 2.73's. Highway mileage is great at almost 16mpg.

08-14-2009, 01:06 PM	#1
JQ-72 Senior Member Join Date: Feb 2001 Location: Killen, Alabama USA Posts: 3,574	73 leaf rear in 72 leaf truck????? Will a leaf spring rear out of a 73-80 model truck (12 bolt) bolt directly into a 72 GMC with leaf springs? I found one with 2.73 gear ratio and wanted to squeeze out an extra mile or 2 out of a gallon of gas - or is it even worth it?? I have 3.08's now. Thanks for any help. Joshua __________________ 72 Cheyenne Super swb. Factory 402/400,tach,posi,AM/FM, A/C. Adding cruise, 8-track 72 Cheyenne Super swb. 350/350 A/C, bucket seats 72 Cheyenne swb. Factory 402/400, tilt, posi, A/C 72 Cheyenne Super K-10. 350/4 spd., posi. 72 Suburban 350/350 tilt, tach, AM/FM, A/C, luggage rack 72 GMC Sierra lwb. 350/350, A/C. medium green/olive interior

08-14-2009, 01:11 PM	#2
Zachary Watts Registered User Join Date: Feb 2009 Location: Pocahontas Ar Posts: 23	Re: 73 leaf rear in 72 leaf truck????? Yes it will bolt in but not directly you will have to relocate one of the shocks hangers to the front of the rearend.

08-14-2009, 01:31 PM	#3
Arizona oldguy Registered User Join Date: Jan 2009 Location: Queen Creek Arizona Posts: 56	Re: 73 leaf rear in 72 leaf truck????? I did this exact swap, I found the rear end still attached to an abandoned 73 long bed, I wanted a lower ratio, and it is a 3:42 I cut the shock mount off of the rear and re-welded it to the front. I also found out that the spacing of the shock mounts are different they are closer together on the 72 I had to use a grade 8 bolt and make it like a shock mount stud, if I had it to do over again I would have cut them both off and re-welded them in the correct place.

08-14-2009, 01:35 PM	#4
capev86 Registered User Join Date: May 2009 Location: Eliot, Maine Posts: 1,314	Re: 73 leaf rear in 72 leaf truck????? 3.08 is kinda lame unless you have a huge motor (like a diesel) and only do light duty towing. going from a 3.08 to 3.73 in my dad's 87 burb 2wd really woke up that 350. 2.73's are good for economy cars because they weigh a lot less and have much shorter tires. on a truck, the larger tires act like an OD. thats why you want a decent gear. 2.73 sounds like slow as molasses to me unless you have a mean engine under the hood. good luck! hint: 3.73 x .7 OD = 2.61 compare 2.61 to straight up 2.73.....more power and lower rpm's. Last edited by capev86; 08-14-2009 at 01:38 PM.

08-14-2009, 04:32 PM	#5
69GMCLonghorn Registered User Join Date: Dec 2006 Location: Porter Ranch, CA Posts: 978	Re: 73 leaf rear in 72 leaf truck????? Yeah, what you have to realize is sometimes you need MORE gears, not just higher gears (numerically lower). As stated above, if you keep some short tires on it, you'll have a lighter rear end and less rolling weight/resistance. Now, in my opinion, lightening up the gears and tires is kind of like putting perfume on a pig; it probably won't net much of a difference. The gear ratio? probably... give it a shot. If it's too high a gear, you know you can always go with lower (numerically higher) gears. I had 2.73's in a '71 Chevelle with 235-60-15 tires on it. I was taching about 2400 at 70 MPH, which was just about right. I used to get between 17 & 19 MPG out of it...

08-14-2009, 05:25 PM	#6
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: 73 leaf rear in 72 leaf truck????? As mentioned that 2.73 may be sorta weak if you intend to do any towing. If you just toodle around mostly on the highway, then it may buy you something but it's not than much of a difference from the 3.08. I went from a 3.54 or something like that to a 3.08 on my 69 and mpg didn't improve much if any. Also, I've towed a 16ft trailer with a tractor and bush hog and didn't have any problems with the 350/350 it's got in it.

08-14-2009, 09:12 PM	#8
PanelDeland I am a Referee of life. Join Date: Jun 2000 Location: Greensboro N.C. Posts: 13,993	Re: 73 leaf rear in 72 leaf truck????? Keep in mind that a 2:73 may lower your RPM's to the point that the engine is out of it's "sweet spot"RPM wise(torque and HP).This may actually worsen your fuel milage.It's could almost be like lugging your engine all the time. __________________ The 47-present Chevrolet and GMC Truck Message Board Network,it's owners,moderators,members,and associates of any type should not be held responsible for my opinion. You can't fix stupid,not even with duct tape. "My appearance is due to the fact that "GOD" does punish you for having too much fun!" Barrett-Jackson has perfected alchemy,they make rust into gold! "You can lead a horse to water but you can't saddle a duck" "Cleverly disguised as a 'Responsible Adult' "Sometimes your Knight in shining armor is just a retard in tinfoil"

08-15-2009, 07:26 AM	#9
JQ-72 Senior Member Join Date: Feb 2001 Location: Killen, Alabama USA Posts: 3,574	Re: 73 leaf rear in 72 leaf truck????? Thanks for all your input. I think you've talked me out of it!! JQ __________________ 72 Cheyenne Super swb. Factory 402/400,tach,posi,AM/FM, A/C. Adding cruise, 8-track 72 Cheyenne Super swb. 350/350 A/C, bucket seats 72 Cheyenne swb. Factory 402/400, tilt, posi, A/C 72 Cheyenne Super K-10. 350/4 spd., posi. 72 Suburban 350/350 tilt, tach, AM/FM, A/C, luggage rack 72 GMC Sierra lwb. 350/350, A/C. medium green/olive interior

08-23-2009, 02:45 PM	#11
glock35ipsc Next project: 1970 K10 Join Date: Feb 2004 Location: Fort Collins, Colo 80524 Posts: 6,685	Re: 73 leaf rear in 72 leaf truck????? So GM got it wrong when they used the 350/TH350/2.73 combo for decades? __________________ *Bob* 1970 Chevy K10 LWB "Goldie" 350/TH350 *next project!!* 1981 Chevy C10 LWB 355/TH350 - My son's truck LS SWAP FORUM! Tons of LS swap info here! PLEASE CLICK HERE TO SUPPORT THE GREATEST BOARD ON THE NET! CLICK HERE FOR THE FAQ INDEX by KRUE Last edited by glock35ipsc; 08-23-2009 at 02:46 PM.

08-23-2009, 06:57 PM	#12
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: 73 leaf rear in 72 leaf truck????? MPG will probably drop. many people on here has done the 700R4/3.07 gear and reported worse MPG. GM got a lot of stuff wrong, and the real tall gear in a truck was one of them.

08-24-2009, 08:24 AM	#13
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,859	Re: 73 leaf rear in 72 leaf truck????? The GM trucks with 2.73s actually drove ok.I`d never run them,but I know of plenty of trucks that satisfied their owners with that combo. __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~



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