Rear diff gear options for 3/4 ton - The 1947 - Present Chevrolet & GMC Truck Message Board Network

p_rubottom · 12-21-2009, 02:55 PM

Hello all, I just picked up a 78 3/4 crew cab. Im wanting to swap gears in the rear diff due to them turning high rpm's at highway speeds. What are my options w/out installing a 700r4 for overdrive?

Thanks

GMC91 · 12-21-2009, 03:26 PM

My 86 3/4 ton has 3.23s in it.

*78 Chevyrado* · 12-21-2009, 03:59 PM

I think GMC91 has the highest gears available... I was looking at that a while back for my 78 c20 and the highest I could find was 3.2x gear set.

GMC91 · 12-21-2009, 04:02 PM

Quote:

Originally Posted by 78 Chevyrado

I think GMC91 has the highest gears available... I was looking at that a while back for my 78 c20 and the highest I could find was 3.2x gear set.

With that being said, how will this truck do with a TH350 on the intersate? Or would my best bet be to find a TH700r and have it rebuilt?

p_rubottom · 12-21-2009, 04:10 PM

Quote:

Originally Posted by 78 Chevyrado

I think GMC91 has the highest gears available... I was looking at that a while back for my 78 c20 and the highest I could find was 3.2x gear set.

Where did you find them at? or what in?

GMC91 · 12-21-2009, 04:20 PM

My 86 came with it when I bought it.

**texassquarebodies** · 12-21-2009, 04:37 PM

Best luck is to check a local wrecking yards and start counting teeth in the rear diff. Also, trucks with the smaller brake drums usually have a numerically lower gear ratio, thats what I was told. I am going to just change out my rear diff. Maybe I will be lucky and find a 3:23!!! I have a couple of them already. Just gotta start counting.

ghettoluxury · 12-21-2009, 05:34 PM

diesels usualy came with the high gears in them.

*78 Chevyrado* · 12-21-2009, 06:06 PM

Quote:

Originally Posted by p_rubottom

Where did you find them at? or what in?

I found it on a web site that sells ring gear/pinion sets. I tried to find it again and I can't find it right now. It was hard to find and I think I only found 1 or 2. Seems anything higher than a 3.73 is uncommon and hard to find.

There's stuff for a 14 bolt 9.5 semi floater axle but you should have a 10.5" 14 bolt full floater in a '78.

Keep hunting, it's out there somewhere.

If i was going to swap trannys at all, I'd put a 700r4 in it. But I wouldn't use a 700r4 with just a stock rebuild if you are going to do much hauling or any towing with it.

munger · 12-21-2009, 07:21 PM

swap to a 700r4 youd like it better a 3 to 1 first gear and od and it has four gears instead of 3 itd be better in the long run

GMC91 · 12-21-2009, 10:50 PM

When I get this truck finished, it aint gonna haul or tow nothing, never gonna see mud either, its strickly a daily driven show truck.

*2tons of fun* · 12-22-2009, 04:52 AM

look in flat bed trucks too, i had a 80's flat bed that was a 454/465 and it had 3.23. granny was good till about 10mph.

i just built a rear with 3.23s for my 70, can wait to get it rolling to see how everything reacts.

79gmcc15 · 12-22-2009, 07:26 AM

why did they put such low ratios on the 3/4? because according to my SPID my 1/2 comes with 3.73's!

12-21-2009, 02:55 PM	#1
p_rubottom Registered User Join Date: Aug 2007 Location: Kansas Posts: 366	Rear diff gear options for 3/4 ton Hello all, I just picked up a 78 3/4 crew cab. Im wanting to swap gears in the rear diff due to them turning high rpm's at highway speeds. What are my options w/out installing a 700r4 for overdrive? Thanks

12-21-2009, 03:26 PM	#2
GMC91 Always trouble Join Date: Jul 2009 Location: Hartsellle, Al Posts: 1,111	Re: Rear diff gear options for 3/4 ton My 86 3/4 ton has 3.23s in it. __________________ 91 Silverado Regular cab, 2wd, swb-project truck 97 Silverado Extended cab, 2wd, swb-driver

12-21-2009, 03:59 PM	#3
78 Chevyrado Account Suspended Join Date: Dec 2007 Location: Unnapreciative, Trollville Posts: 2,079	Re: Rear diff gear options for 3/4 ton I think GMC91 has the highest gears available... I was looking at that a while back for my 78 c20 and the highest I could find was 3.2x gear set. Last edited by 78 Chevyrado; 12-21-2009 at 04:00 PM.

12-21-2009, 04:20 PM	#6
GMC91 Always trouble Join Date: Jul 2009 Location: Hartsellle, Al Posts: 1,111	Re: Rear diff gear options for 3/4 ton My 86 came with it when I bought it. __________________ 91 Silverado Regular cab, 2wd, swb-project truck 97 Silverado Extended cab, 2wd, swb-driver

12-21-2009, 04:37 PM	#7
texassquarebodies Truck Nut Join Date: Jun 2007 Location: Wills Point, Texas Posts: 407	Re: Rear diff gear options for 3/4 ton Best luck is to check a local wrecking yards and start counting teeth in the rear diff. Also, trucks with the smaller brake drums usually have a numerically lower gear ratio, thats what I was told. I am going to just change out my rear diff. Maybe I will be lucky and find a 3:23!!! I have a couple of them already. Just gotta start counting. __________________ Tony Kee Texas Square Bodies

12-21-2009, 05:34 PM	#8
ghettoluxury Registered User Join Date: Apr 2009 Location: Waynesfield Ohio Posts: 1,991	Re: Rear diff gear options for 3/4 ton diesels usualy came with the high gears in them. __________________ 1949 Five Window, 1973 Step Side, 2000 Z71

12-21-2009, 07:21 PM	#10
munger Registered User Join Date: Nov 2009 Location: goodland,ks Posts: 217	Re: Rear diff gear options for 3/4 ton swap to a 700r4 youd like it better a 3 to 1 first gear and od and it has four gears instead of 3 itd be better in the long run __________________ mitch 87 chevy short box 4wd tbi 350/built 700r4/np208 3.73s

12-21-2009, 10:50 PM	#11
GMC91 Always trouble Join Date: Jul 2009 Location: Hartsellle, Al Posts: 1,111	Re: Rear diff gear options for 3/4 ton When I get this truck finished, it aint gonna haul or tow nothing, never gonna see mud either, its strickly a daily driven show truck. __________________ 91 Silverado Regular cab, 2wd, swb-project truck 97 Silverado Extended cab, 2wd, swb-driver

12-22-2009, 04:52 AM	#12
2tons of fun Account Suspended Join Date: Jan 2009 Location: galt ca, Posts: 749	Re: Rear diff gear options for 3/4 ton look in flat bed trucks too, i had a 80's flat bed that was a 454/465 and it had 3.23. granny was good till about 10mph. i just built a rear with 3.23s for my 70, can wait to get it rolling to see how everything reacts.

12-22-2009, 07:26 AM	#13
79gmcc15 Gettin It One Pice At A Time Join Date: Oct 2004 Location: everett, washington Posts: 2,688	Re: Rear diff gear options for 3/4 ton why did they put such low ratios on the 3/4? because according to my SPID my 1/2 comes with 3.73's! __________________ RIP ELJAY Work In Progress 1979 GMC C1500 Trailering Special. Getting the truck done, one piece at a time. “You can tell a lot about a fellow’s character by his way of eating jellybeans.”Ronald Reagan “I don’t know the key to success but I know the key to failure is trying to please everybody.” Bill Cosby



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