rear end ratio - The 1947 - Present Chevrolet & GMC Truck Message Board Network

norwegianc10 · 07-18-2008, 07:14 AM

Anyone have an idea what rear-end I have got, and what rear-end I need?
Now I am doing 50mph at 1900rpm and I would really like to lower that to like 1400-1500rpm. I have got 235/60R15 tires and th400 tranny. I know the best thing would be to get an overdrive tranny, but I live in Norway, and they are really hard to get hold on here. Thanks in advance

**LockDoc** · 07-18-2008, 08:38 AM

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There are several ways to tell your current ratio. The one I usually use is the letters stamped on the housing. I have included a couple of pictures that show the location of the letters. (use the first two or three letters) When you have the code go to the following web site and look it up in the axle section. (you have to select the year of your truck)

http://www.americanclassic.com/paint_test.asp

Another good way is to take the cover off of the rear housing and look at the numbers stamped on the gears. The good thing about using this method is that you put fresh gear oil in the rear end to replace the 40 year old oil that is in there now. Below are the numbers for the different ratios. The last two numbers in each row are the ones that will be stamped into the gears.

Rear Axle Ratios

Ratio 2.56:1 - Tooth Combination 41,16
Ratio 2.73:1 - Tooth Combination 41,15
Ratio 3.07:1 - Tooth Combination 43,14
Ratio 3.08:1 - Tooth Combination 40,13
Ratio 3.08:1 - Tooth Combination 37,12
Ratio 3.31:1 - Tooth Combination 43,13
Ratio 3.36:1 - Tooth Combination 37,11
Ratio 3.42:1 - Tooth Combination 41,12
Ratio 3.55:1 - Tooth Combination 39,11
Ratio 3.73:1 - Tooth Combination 41,11
Ratio 4.10:1 - Tooth Combination 41,10
Ratio 4.11:1 - Tooth Combination 37,9

LockDoc

**steelhorse** · 07-18-2008, 08:49 AM

Jack it up put a mark on the drive shaft an the tire.Count how many revolutions the driveshaft turns to make the wheel turn 1 time.A quick and easy way to raise the gear ratio whould be to switch to a taller tire,but you will then need to switch the speedo gear.

samwise68 · 07-18-2008, 11:25 AM

by using a tire size calculator, and an rpm calculator, you have around a 3.07/3.08 rear end.

Thats assuming your 235/60 is 26 inches tall. And given the Th400 has a 1:1 final drive...yeah.

Honestly, I don't know if you'd want to go much lower. a 2.73 would really lug you down out of the hole. And thats about it for dropping the RPMs. a taller tire would accomplish the trick too, but minimally. I dunno.

I'm running 3.07 gears, with a 235/75 tire. and she'd doggy out of the hole, requiring some clutch to get going.

Just my 2 cents.

-Sam.

norwegianc10 · 07-18-2008, 03:54 PM

I will check out my rear axle tomorrow, thanks for quick reply. Great board by the way

piecesparts · 07-18-2008, 04:18 PM

I calculate that you have a 2.95 gear ratio in your rear end. That is using a 26.1" tire diameter, a tranny output ratio of 1.0. The calculation is easy.

Take your RPMs and multipy them by tire diameter. Next multiply the MPH by tranny output ration and then multiply by 336. This last number is divided INTO the first set of numbers to get the 2.95 gear ratio. Equivalent to a 3.07 moreor less.

RPM X Tire Diameter divided by MPH X Tans output X 336 = 2.95178

My thinking is that you will not like the lower RPMs very much. That will make your truck really a wimp at the street light and the gas mileage make sucketh worse.

We took my son's 86 from a 3.07 gear ratio to a 3.73 (with an OD tranny) and increased our mileage buy2 MPG, due to getting the engine into the power curve where it did not have to work so hard. He now runs at about 2500 RPM at 70 MPH and around 2100 at 55 MPH.

cruiserman · 07-18-2008, 06:14 PM

You can put larger tires on the truck for an overdrive effect.

**WorkinLonghorn** · 07-18-2008, 08:08 PM

A taller gear ratio or bigger tire may not help.It depends on what motor config. you are running. I had a '69 1/2 ton with 3.07s and 33" tires and it ran 1875 RPM (manual trans) at 60 MPH and got 10 MPG on the freeway.My current (similar engine-350) has 3.73 rear and 31" and runs 2500 at 60 but gets 15.5 MPG.If your motor is most efficient and gets peak torque at 2600,then running at 1500-1800 may not be ideal mileage-wise and it could be a dog off the line,and I don't mean a grayhound.More like a bassett.

Longhorn Man · 07-18-2008, 08:19 PM

those RPMs would be a 3.07 gear, and in all reality, if you drop your RPMs to what you are wanting, your fuel consumption will go up instead of down.

07-18-2008, 07:14 AM	#1
norwegianc10 Registered User Join Date: Jun 2008 Location: Norway Posts: 90	rear end ratio Anyone have an idea what rear-end I have got, and what rear-end I need? Now I am doing 50mph at 1900rpm and I would really like to lower that to like 1400-1500rpm. I have got 235/60R15 tires and th400 tranny. I know the best thing would be to get an overdrive tranny, but I live in Norway, and they are really hard to get hold on here. Thanks in advance

07-18-2008, 08:38 AM	#2
LockDoc The Older Generation Join Date: Aug 2006 Location: Montezuma, Iowa Posts: 25,772	Re: rear end ratio - There are several ways to tell your current ratio. The one I usually use is the letters stamped on the housing. I have included a couple of pictures that show the location of the letters. (use the first two or three letters) When you have the code go to the following web site and look it up in the axle section. (you have to select the year of your truck) http://www.americanclassic.com/paint_test.asp Another good way is to take the cover off of the rear housing and look at the numbers stamped on the gears. The good thing about using this method is that you put fresh gear oil in the rear end to replace the 40 year old oil that is in there now. Below are the numbers for the different ratios. The last two numbers in each row are the ones that will be stamped into the gears. Rear Axle Ratios Ratio 2.56:1 - Tooth Combination 41,16 Ratio 2.73:1 - Tooth Combination 41,15 Ratio 3.07:1 - Tooth Combination 43,14 Ratio 3.08:1 - Tooth Combination 40,13 Ratio 3.08:1 - Tooth Combination 37,12 Ratio 3.31:1 - Tooth Combination 43,13 Ratio 3.36:1 - Tooth Combination 37,11 Ratio 3.42:1 - Tooth Combination 41,12 Ratio 3.55:1 - Tooth Combination 39,11 Ratio 3.73:1 - Tooth Combination 41,11 Ratio 4.10:1 - Tooth Combination 41,10 Ratio 4.11:1 - Tooth Combination 37,9 LockDoc Attached Images __________________ Leon Locksmith, Specializing In Antique Trucks, Automobiles, & Motorcycles (My Dually Pickup Project Thread) http://67-72chevytrucks.com/vboard/s...d.php?t=829820 - Last edited by LockDoc; 11-16-2009 at 05:06 PM.

07-18-2008, 08:49 AM	#3
steelhorse Senior Member Join Date: Apr 2008 Location: Franklinton NC Posts: 1,309	Re: rear end ratio Jack it up put a mark on the drive shaft an the tire.Count how many revolutions the driveshaft turns to make the wheel turn 1 time.A quick and easy way to raise the gear ratio whould be to switch to a taller tire,but you will then need to switch the speedo gear.

07-18-2008, 03:54 PM	#5
norwegianc10 Registered User Join Date: Jun 2008 Location: Norway Posts: 90	Re: rear end ratio I will check out my rear axle tomorrow, thanks for quick reply. Great board by the way

07-18-2008, 04:18 PM	#6
piecesparts Parts and more parts Join Date: Nov 2003 Location: Lebo, Kansas (middle of nowhere Posts: 6,821	Re: rear end ratio I calculate that you have a 2.95 gear ratio in your rear end. That is using a 26.1" tire diameter, a tranny output ratio of 1.0. The calculation is easy. Take your RPMs and multipy them by tire diameter. Next multiply the MPH by tranny output ration and then multiply by 336. This last number is divided INTO the first set of numbers to get the 2.95 gear ratio. Equivalent to a 3.07 moreor less. RPM X Tire Diameter divided by MPH X Tans output X 336 = 2.95178 My thinking is that you will not like the lower RPMs very much. That will make your truck really a wimp at the street light and the gas mileage make sucketh worse. We took my son's 86 from a 3.07 gear ratio to a 3.73 (with an OD tranny) and increased our mileage buy2 MPG, due to getting the engine into the power curve where it did not have to work so hard. He now runs at about 2500 RPM at 70 MPH and around 2100 at 55 MPH. Last edited by piecesparts; 07-18-2008 at 04:24 PM.

07-18-2008, 06:14 PM	#7
cruiserman Registered User Join Date: Dec 2006 Location: Idaho Posts: 210	Re: rear end ratio You can put larger tires on the truck for an overdrive effect. __________________ 72 C2500 LWB 402 Sierra Grande 05 Odyssey 08 Tundra

07-18-2008, 08:08 PM	#8
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,883	Re: rear end ratio A taller gear ratio or bigger tire may not help.It depends on what motor config. you are running. I had a '69 1/2 ton with 3.07s and 33" tires and it ran 1875 RPM (manual trans) at 60 MPH and got 10 MPG on the freeway.My current (similar engine-350) has 3.73 rear and 31" and runs 2500 at 60 but gets 15.5 MPG.If your motor is most efficient and gets peak torque at 2600,then running at 1500-1800 may not be ideal mileage-wise and it could be a dog off the line,and I don't mean a grayhound.More like a bassett. __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme.

07-18-2008, 08:19 PM	#9
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Re: rear end ratio those RPMs would be a 3.07 gear, and in all reality, if you drop your RPMs to what you are wanting, your fuel consumption will go up instead of down.



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