Rear End Gear Ratio - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**78 Big10 454** · 09-07-2008, 01:47 AM

My P/U has a 2.73 posi rear end. I need you, the experts, to recommend a good rear end gear ratio for highway cruising. Any help is greatly appreciated!

Low84 · 09-07-2008, 03:09 AM

I'm by no means an expert, but I'm sure they will need a little more info. Like total wheel/tire diameter and what transmission you're running.

BLE 'BURBAN · 09-07-2008, 03:25 AM

The lower the number the higher the gear ratio is. Thusly if you want good highway cruising gears your 2:73's are a real good set as is. Unless that's a typo and you meant 3:73's instead. Then yeah I'd have to say get some lower number'd gears like 2:91 - 3;07's You'll also have to re-gear your speedometer gearing to match otherwise your speedo's gonna read wrong

**Captkaos** · 09-08-2008, 06:11 PM

3.42 is about the best you can get for a compromise of good acceleration and cruise speed.

slugish · 09-08-2008, 06:52 PM

Ditto to captkaos- best of both worlds 3:42 for a 3 speed trans
3:73 for an overdrive 4 speed

Of course extreme tire height differences from stock will have an effect on this as well. In other words if you are running a 31-332 in rear tire , you will be ok with a 3.73 in a3 speed trans.

**78 Big10 454** · 09-08-2008, 08:14 PM

Many thanks to all of your replies. I have a great deal of respect for the participants of this board. You're the greatest!

**N2TRUX** · 09-08-2008, 08:26 PM

Since you didn't provide any more details, I'll surmise by your screen name that we are talking about a big block powered truck. With that piece of information, I will assume you are running a TH400. Based on that theory, I would suggest a 3.08 if you want optimal mileage, and a 3.42 is you are willing to sacrifice 1-2 mpg for some additional torque.

piecesparts · 09-09-2008, 04:49 PM

What do you have for tire size and tranny in the truck? This info will support a calculation of RPMs on the highway. My guess is the 2.73 is killing you on the hills and the city.

Pyrotechnic · 09-09-2008, 07:27 PM

With a 28" tire, 3.42's are an awesome gear for mixed city and highway driving.

However, if you have to do long highway trips, in the order of several hours on a regular basis, then your RPM's are going to be a bit uncomfortable on the highway. I would recommend 3.42's if you only have to do a few miles of highway driving a day.

If you do a lot of long highway trips, the 2.73's are probably the best thing for you if you want to keep RPM's down. You really can't expect them to be fun or accelerate quickly though.

If you want the best of both worlds, then you need an overdrive transmission.

piecesparts · 09-09-2008, 09:46 PM

Do you use your cruise control, and do you have hills that you drive in, where you live? The 3.42 or a 3.73 will give you a good pull in the hills and the cruise will not kick down as much as it would on a 3.08 gear or higher. I talk from experience, I had a 3.42 gear in my truck and changed it to the 3.73 to get the acceleration on the street. I run at 2500 RPM at 70 MPH with an OD tranny. If that were a standard auto it would be about 300 to 400 RPMs higher. My DD truck (91 Silverado) runs at 2200 RPM with an OD tranny and 3.42 gears (both trucks have 28" tires on the rear).

tecky11 · 01-18-2009, 04:38 AM

HI, i don't know if you can help me or not,but i have a 1972 International harvester 1210 p/u(3/4 ton) and i have no idea on what the rear gear ratio is. Can you help me out?

D13 · 01-18-2009, 08:42 AM

The other thing to look at is the carrier. Depending on which rear end (10 or 12 bolt) is under your truck, the normal gear set carrier change is at 3.23 or 3.43. This may involve swapping out your carrier (ie the posi unit) to get the lower gears. Often you can get a gear set that avoids this by using a spacer to correct the ring gear position, but they are not as strong as the correct carrier. Might want to check Summit for the available selection.

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James McClure · 01-18-2009, 10:33 AM

I run 3.08 gears, 31" tires and a rock crusher 4 speed. I'm turning about 2200 RPM at 70. The eng is a 350 with a quad and I get about 17 mpg. I like the 3.08 gear set. If I need power I got that real low 1st gear as well as deeper reduction going into 4wheel low range, and the lower ratio gives decent fuel economy. jim

LONGHAIR · 01-18-2009, 11:20 AM

Quote:

Originally Posted by James McClure

I run 3.08 gears, 31" tires and a rock crusher 4 speed. I'm turning about 2200 RPM at 70. The eng is a 350 with a quad and I get about 17 mpg. I like the 3.08 gear set. If I need power I got that real low 1st gear as well as deeper reduction going into 4wheel low range, and the lower ratio gives decent fuel economy. jim

Rock Crusher? real low first gear? I am confused....
A muncie (M-22 rock crusher) is a high geared close ratio car transmission with a 2.20-1 first gear. IMO they would suck in a big heavy truck in the first place, but I have never heard of anyone adapting one to a 4x4?

James McClure · 01-18-2009, 10:39 AM

tecky, isn't there an International board? Well anyways, gears is gears. Do this, count the teeth on the pinion gear, count the teeth on the ring gear. Divide the ring by the pinion and thats the ratio. Example, pinion gear, 15 teeth, ring gear, 41 teeth. Divide 41 by 15 and you get 2:73. Make sense to you?? jim

tecky11 · 01-18-2009, 11:29 AM

I have a 10 bolte full floating rear. But I don't think it is a posi rear . How would I fins this out? I also need all 4 drums can anyone tell me where I can get them without spending a fortune on them? Any help would be appreciated

LONGHAIR · 01-18-2009, 11:40 AM

Quote:

Originally Posted by tecky11

I have a 10 bolte full floating rear. But I don't think it is a posi rear . How would I fins this out? I also need all 4 drums can anyone tell me where I can get them without spending a fortune on them? Any help would be appreciated

Although there are no specific rules against "hi-jacking" someone's thread, it is not a good idea....especially for a first time poster.
However it is against the rules to post a "want to buy" messege.

Quote:

XIV. As of October 15th 2005 subscribing members will be the only ones permitted to post for sale and want to buy ads on any classified section of this site. All other rules of buying and selling still apply. 67-72chevytrucks.com, its administrators or staff are not responsible for any transactions on or off the forums. We will work hard to ensure you a smooth transaction, and help in the event you may need it, but all for sale and trade areas are buyer beware. Any persons who have misrepresented an item, lie about an item, fail to deliver, or are in violation of any of the rules will forfeit there subscription fees and be banned from 67-72chevytrucks.com

I would assume that an International truck would have a Dana 44 rear end though I don't have any idea about parts retailers for INT parts.

justcuz · 01-22-2009, 02:21 AM

Quote:

Originally Posted by tecky11

I have a 10 bolte full floating rear. But I don't think it is a posi rear . How would I fins this out? I also need all 4 drums can anyone tell me where I can get them without spending a fortune on them? Any help would be appreciated

Go to the Binder Bulletin.com, they specialize in International trucks and can give you better answers.

James McClure · 01-18-2009, 06:48 PM

Long hair. I think there is a mix up in terms. We all up here call the 117MM a rock crusher. I think you all refer to it as a 465?? Whenever I went to parts they wanted it in MM. Hope that clears it up. jim

Justins76 · 01-20-2009, 12:25 AM

I have 3.42s and 29" tires and when you get out on the highway where its a 75mph limit, it really has the engine revved. For lots of highway the 2.73 should be good.

piecesparts · 01-20-2009, 06:25 PM

Quote:

Originally Posted by Justins76

I have 3.42s and 29" tires and when you get out on the highway where its a 75mph limit, it really has the engine revved. For lots of highway the 2.73 should be good.

What do you consider having the engine really revved? My truck at 75 MPH with a 28 " tire, a 700R4 tranny and 3.42 gears runs round 2200 RPM Right where my power band is the best.

Do you have an OD tranny?

Justins76 · 01-22-2009, 01:36 AM

Little above 3000RPM. I have a TH350 so 3rd is 1:1. With a 700r4 in OD its .7 so its 30% less, plus you have the tc lockup. I'm looking into a 700r4 swap someday.

Quote:

Originally Posted by piecesparts

What do you consider having the engine really revved? My truck at 75 MPH with a 28 " tire, a 700R4 tranny and 3.42 gears runs round 2200 RPM Right where my power band is the best.

Do you have an OD tranny?

69DISASTER · 04-01-2009, 01:12 AM

I just bought a 1972 C1500 and discovered it has an SM465 in it and man Does this engine rev on the highway!!! I have a 69 chassis with a 350ci and th400. How much is involved in swapping this out. Ie crossmembers, driveshafts etc? Not really ready to do swap right now any thoughts on maybe gear ratio change, like 273'3 or higher?

texacokie · 05-04-2009, 12:57 PM

I used to have a truck with 2:73 gears and man it got great gas mileage!

billnorman · 05-05-2009, 07:03 AM

Some of the Big 10's had 3:40 rear, 12 bolt. I have one I put in my '85 out of a Big 10. I counted it three times , 10 and 34 teeth.

09-07-2008, 01:47 AM	#1
78 Big10 454 78 Big10 454 Join Date: Jul 2008 Location: Pismo Beach, CA Posts: 838	Rear End Gear Ratio My P/U has a 2.73 posi rear end. I need you, the experts, to recommend a good rear end gear ratio for highway cruising. Any help is greatly appreciated!

09-07-2008, 03:09 AM	#2
Low84 Registered User Join Date: Oct 2004 Location: Roma,TX Posts: 774	Re: Rear End Gear Ratio I'm by no means an expert, but I'm sure they will need a little more info. Like total wheel/tire diameter and what transmission you're running. __________________ 1978 stepside C10 build: http://67-72chevytrucks.com/vboard/s...d.php?t=468265

09-07-2008, 03:25 AM	#3
BLE 'BURBAN In the Forgotten far North. Join Date: Jul 2007 Location: Anchorage, Alaska Posts: 2,627	Re: Rear End Gear Ratio The lower the number the higher the gear ratio is. Thusly if you want good highway cruising gears your 2:73's are a real good set as is. Unless that's a typo and you meant 3:73's instead. Then yeah I'd have to say get some lower number'd gears like 2:91 - 3;07's You'll also have to re-gear your speedometer gearing to match otherwise your speedo's gonna read wrong __________________ 1987 R3500 CREW CAB DUALLY (BIG RED)Acquired 06/12/2015 1990 chevy suburban V2500 5.7L My cluster Mods-Nov 2007 overhead console Stereo install Round 2 Aug 2009 Heated/turn signal mirror upgrade

09-08-2008, 06:11 PM	#4
Captkaos www.73-87chevytrucks.com Join Date: Dec 1999 Location: Birmingham, AL Posts: 6,935	Re: Rear End Gear Ratio 3.42 is about the best you can get for a compromise of good acceleration and cruise speed. __________________ Chris Lucas 1973 Chevy C-10 1978 GMC Jimmy (2WD) - SOLD 1987 R10 twin turbo LS 1991 R3500 SRW CrewCab 1985 K5 diesel swapped project 1989 K5 2WD conversion w/ Vette susp Project Captkaos Customs 73-87chevytrucks.com

09-08-2008, 06:52 PM	#5
slugish Slugish Join Date: Jan 2005 Location: Earlysville, Va. Posts: 1,024	Re: Rear End Gear Ratio Ditto to captkaos- best of both worlds 3:42 for a 3 speed trans 3:73 for an overdrive 4 speed Of course extreme tire height differences from stock will have an effect on this as well. In other words if you are running a 31-332 in rear tire , you will be ok with a 3.73 in a3 speed trans.

09-08-2008, 08:14 PM	#6
78 Big10 454 78 Big10 454 Join Date: Jul 2008 Location: Pismo Beach, CA Posts: 838	Re: Rear End Gear Ratio Many thanks to all of your replies. I have a great deal of respect for the participants of this board. You're the greatest!

09-08-2008, 08:26 PM	#7
N2TRUX Happy to be here Join Date: Mar 2000 Location: Cypress, TX Posts: 39,023	Re: Rear End Gear Ratio Since you didn't provide any more details, I'll surmise by your screen name that we are talking about a big block powered truck. With that piece of information, I will assume you are running a TH400. Based on that theory, I would suggest a 3.08 if you want optimal mileage, and a 3.42 is you are willing to sacrifice 1-2 mpg for some additional torque. __________________ Follow me on Facebook and Instagram @N2trux.com Articles- "Jake" the 84 to 74 crewcab "Elwood" the77_Remix 85 GMC Sierra "Scarlett" "Refining Sierra"

09-09-2008, 04:49 PM	#8
piecesparts Parts and more parts Join Date: Nov 2003 Location: Lebo, Kansas (middle of nowhere Posts: 6,821	Re: Rear End Gear Ratio What do you have for tire size and tranny in the truck? This info will support a calculation of RPMs on the highway. My guess is the 2.73 is killing you on the hills and the city.

09-09-2008, 07:27 PM	#9
Pyrotechnic Registered User Join Date: Feb 2007 Location: Austin, TX Posts: 3,930	Re: Rear End Gear Ratio With a 28" tire, 3.42's are an awesome gear for mixed city and highway driving. However, if you have to do long highway trips, in the order of several hours on a regular basis, then your RPM's are going to be a bit uncomfortable on the highway. I would recommend 3.42's if you only have to do a few miles of highway driving a day. If you do a lot of long highway trips, the 2.73's are probably the best thing for you if you want to keep RPM's down. You really can't expect them to be fun or accelerate quickly though. If you want the best of both worlds, then you need an overdrive transmission. __________________ 1977 GMC Sierra Grande

09-09-2008, 09:46 PM	#10
piecesparts Parts and more parts Join Date: Nov 2003 Location: Lebo, Kansas (middle of nowhere Posts: 6,821	Re: Rear End Gear Ratio Do you use your cruise control, and do you have hills that you drive in, where you live? The 3.42 or a 3.73 will give you a good pull in the hills and the cruise will not kick down as much as it would on a 3.08 gear or higher. I talk from experience, I had a 3.42 gear in my truck and changed it to the 3.73 to get the acceleration on the street. I run at 2500 RPM at 70 MPH with an OD tranny. If that were a standard auto it would be about 300 to 400 RPMs higher. My DD truck (91 Silverado) runs at 2200 RPM with an OD tranny and 3.42 gears (both trucks have 28" tires on the rear).

01-18-2009, 04:38 AM	#11
tecky11 Registered User Join Date: Jan 2009 Location: phoenix,AZ Posts: 451	Re: Rear End Gear Ratio HI, i don't know if you can help me or not,but i have a 1972 International harvester 1210 p/u(3/4 ton) and i have no idea on what the rear gear ratio is. Can you help me out?

01-18-2009, 10:33 AM	#13
James McClure One shot, one kill. Join Date: Dec 2007 Location: Saratoga Springs NY Posts: 859	Re: Rear End Gear Ratio I run 3.08 gears, 31" tires and a rock crusher 4 speed. I'm turning about 2200 RPM at 70. The eng is a 350 with a quad and I get about 17 mpg. I like the 3.08 gear set. If I need power I got that real low 1st gear as well as deeper reduction going into 4wheel low range, and the lower ratio gives decent fuel economy. jim __________________ 1984 K20 350M engine with 465,000 miles. Well, it's finally done!! Almost 2 years of work, but it was really worth the effort. Little stuff left to do is mount winch, wet sand & buff out and build belly pan\running boards. Body work takes lots and LOTS of beer!!!! God, Guts and Guns made America and God, Guts and Guns will preserve it! The worst thing you could do is get into my sights, but that don't matter, you'll never know it, cause you'll never hear the shotJim or Paladin whichever you choose.

01-18-2009, 10:39 AM	#15
James McClure One shot, one kill. Join Date: Dec 2007 Location: Saratoga Springs NY Posts: 859	Re: Rear End Gear Ratio tecky, isn't there an International board? Well anyways, gears is gears. Do this, count the teeth on the pinion gear, count the teeth on the ring gear. Divide the ring by the pinion and thats the ratio. Example, pinion gear, 15 teeth, ring gear, 41 teeth. Divide 41 by 15 and you get 2:73. Make sense to you?? jim __________________ 1984 K20 350M engine with 465,000 miles. Well, it's finally done!! Almost 2 years of work, but it was really worth the effort. Little stuff left to do is mount winch, wet sand & buff out and build belly pan\running boards. Body work takes lots and LOTS of beer!!!! God, Guts and Guns made America and God, Guts and Guns will preserve it! The worst thing you could do is get into my sights, but that don't matter, you'll never know it, cause you'll never hear the shotJim or Paladin whichever you choose.

01-18-2009, 11:29 AM	#16
tecky11 Registered User Join Date: Jan 2009 Location: phoenix,AZ Posts: 451	Re: Rear End Gear Ratio I have a 10 bolte full floating rear. But I don't think it is a posi rear . How would I fins this out? I also need all 4 drums can anyone tell me where I can get them without spending a fortune on them? Any help would be appreciated

01-18-2009, 06:48 PM	#19
James McClure One shot, one kill. Join Date: Dec 2007 Location: Saratoga Springs NY Posts: 859	Re: Rear End Gear Ratio Long hair. I think there is a mix up in terms. We all up here call the 117MM a rock crusher. I think you all refer to it as a 465?? Whenever I went to parts they wanted it in MM. Hope that clears it up. jim __________________ 1984 K20 350M engine with 465,000 miles. Well, it's finally done!! Almost 2 years of work, but it was really worth the effort. Little stuff left to do is mount winch, wet sand & buff out and build belly pan\running boards. Body work takes lots and LOTS of beer!!!! God, Guts and Guns made America and God, Guts and Guns will preserve it! The worst thing you could do is get into my sights, but that don't matter, you'll never know it, cause you'll never hear the shotJim or Paladin whichever you choose.



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01-20-2009, 12:25 AM	#20
Justins76 OutOfMyMind & I'llNeverKnowWhy Join Date: Apr 2004 Location: Another world, some other time Posts: 672	Re: Rear End Gear Ratio I have 3.42s and 29" tires and when you get out on the highway where its a 75mph limit, it really has the engine revved. For lots of highway the 2.73 should be good. __________________ -Justin-Out of my mind and I'll never know why 1976 Chevrolet Silverado C10 350 4bbl / TH350 Big10 longbed 1986 Pontiac Firebird T-Top 5.0L TPI 700R4 WS6 1992 Pontiac Firebird T-Top 5.0L TBI 700R4 My cardomain page I don't like to save, it's more fun to spend!!

04-01-2009, 01:12 AM	#23
69DISASTER Registered User Join Date: Oct 2008 Location: CALGARY, ALBERTA Posts: 62	Re: Rear End Gear Ratio I just bought a 1972 C1500 and discovered it has an SM465 in it and man Does this engine rev on the highway!!! I have a 69 chassis with a 350ci and th400. How much is involved in swapping this out. Ie crossmembers, driveshafts etc? Not really ready to do swap right now any thoughts on maybe gear ratio change, like 273'3 or higher?

05-04-2009, 12:57 PM	#24
texacokie Registered User Join Date: Nov 2008 Location: oklahoma city oklahoma Posts: 108	Re: Rear End Gear Ratio I used to have a truck with 2:73 gears and man it got great gas mileage!

05-05-2009, 07:03 AM	#25
billnorman Registered User Join Date: Mar 2009 Location: glen burnie md Posts: 174	Re: Rear End Gear Ratio Some of the Big 10's had 3:40 rear, 12 bolt. I have one I put in my '85 out of a Big 10. I counted it three times , 10 and 34 teeth.

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