rear end gear ration for t56 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

67seaten · 02-16-2017, 03:19 AM

hey guys , wondering what the best gear ratio that i should be using , running a ls1 with a t56, thinking about running a 27 tall tire

**rob32472** · 02-16-2017, 05:29 AM

What motor/cam combo are you planning to go with?

trkfrk · 02-16-2017, 01:41 PM

Use this gear ratio calculator. http://www.grimmjeeper.com/gears.html . It really helped me and takes the guessing out of the equation. You can play around with it. I went from 3:73 to 4:56's. I tach 2,000 RPM/80MPH/6th gear and average 17MPG.

67ChevyRedneck · 02-16-2017, 01:56 PM

With OD you don't really want anything higher (numerically lower) than a 3.73 in these heavy old aerodynamic bricks. My truck is happy with a 700R and 3.73 and turns around 2300 RPM at 75 mph. I believe the T56 has an even greater OD ratio (like 0.63?) vs the 0.7 of the 700 will lower the rpms even more. A 4:11 would probably be a good match as well if you want more low end grunt and still be good on the highway with the 0.63 OD.

67seaten · 02-16-2017, 03:06 PM

running a ls1 with a mild cam , gonna be alot of street driving some highway driving , i have a set 456's on the shelf for another project . if they do work well with this set up i will use them ,i didn't want to put it in they didn't work then end up taking it out

sangell · 02-16-2017, 03:29 PM

What is your t56 out of? They had different ratios, the most common behind the LS motors are the F-body version with a .50 6th. In my son's truck we have a CTSV T56 with the .56 6th gear and 27" tires. I put a 4.56 in it. With the low rear gear and high OD ratios you need to watch out for drive shaft RPM. His truck was a long bed and I tried to run a one piece shaft that didn't work out. With 27" tires, .56 6th gear it was 2400 rpm at 75mph which put the drive shaft at 4300 rpm and it was not happy. The drive shaft is a 4"x.083 wall steel piece. We have since cut the truck down to a shortbed so that makes the driveshaft 12" shorter and I'm dropping to either 4.10s or 3.73s. I think the 4.10s are the way to go but the 3.73s help so much with the drive shaft RPM.

sangell · 02-16-2017, 03:37 PM

Quote:

Originally Posted by trkfrk

Use this gear ratio calculator. http://www.grimmjeeper.com/gears.html . It really helped me and takes the guessing out of the equation. You can play around with it. I went from 3:73 to 4:56's. I tach 2,000 RPM/80MPH/6th gear and average 17MPG.

So are you running roughly a 30" tall tire?

trkfrk · 02-16-2017, 05:17 PM

Quote:

Originally Posted by sangell

So are you running roughly a 30" tall tire?

Yes, 295/45/20 which equals 30.45" OD.

Heavymetl · 02-16-2017, 05:42 PM

Driveshaft speed is the killer here. I'd say 4.10s would work, but look into aluminum for the driveshaft material.

I run 4.30s in my T56 Camaro, but it's got a relatively short 3.5" diameter aluminum driveshaft and runs smooth as butter at any speed.

sangell · 02-16-2017, 06:58 PM

When you get to big dia thin wall steel they have a equal or higher critical speed than an AL shaft does.

Mike C · 02-17-2017, 09:08 AM

Some good advice above. My recommendation for the .5 OD T56 would be 4.56 at a minimum. Especially if you are running a carburetor still. FI allows for even lower cruise speeds and good AF ratio.

I wouldn't consider 4.88 to be out of line with a 30/31" tire either.

sangell · 02-17-2017, 09:50 AM

This is from Mark Williams

67seaten · 02-17-2017, 06:52 PM

thanks guys , its out of a camaro i am gonna stay efi , 27-29ish tall tire is what i am planning on running , aluminum drive shaft will be the way ill go , i will need a custom made once everything is together

02-16-2017, 03:19 AM	#1
67seaten Registered User Join Date: Nov 2016 Location: winnipeg Posts: 62	rear end gear ration for t56 hey guys , wondering what the best gear ratio that i should be using , running a ls1 with a t56, thinking about running a 27 tall tire __________________ mike

02-16-2017, 05:29 AM	#2
rob32472 Registered User Join Date: May 2007 Location: dallas ga Posts: 1,503	Re: rear end gear ration for t56 What motor/cam combo are you planning to go with?

02-16-2017, 01:41 PM	#3
trkfrk Registered User Join Date: Jul 2010 Location: Las Vegas, NV Posts: 969	Re: rear end gear ration for t56 Use this gear ratio calculator. http://www.grimmjeeper.com/gears.html . It really helped me and takes the guessing out of the equation. You can play around with it. I went from 3:73 to 4:56's. I tach 2,000 RPM/80MPH/6th gear and average 17MPG. __________________ Mark 1972 C10 SWB LS1/T56 6 speed trans/4:56 posi with 5.5"-7" static drop/Boss 338 Wheels 18"x20" My build thread: http://67-72chevytrucks.com/vboard/s...d.php?t=424609. My first start up http://youtu.be/R899YQ1OcjU Last edited by trkfrk; 02-16-2017 at 01:46 PM.

02-16-2017, 01:56 PM	#4
67ChevyRedneck Hittin E-Z Street on Mud Tires Join Date: Nov 2004 Location: Greenville, SC Posts: 23,090	Re: rear end gear ration for t56 With OD you don't really want anything higher (numerically lower) than a 3.73 in these heavy old aerodynamic bricks. My truck is happy with a 700R and 3.73 and turns around 2300 RPM at 75 mph. I believe the T56 has an even greater OD ratio (like 0.63?) vs the 0.7 of the 700 will lower the rpms even more. A 4:11 would probably be a good match as well if you want more low end grunt and still be good on the highway with the 0.63 OD. __________________ Jesse James 1967 C10 SWB Stepside: 350/700R4/3.73 1965 Ford Mustang: 289/T5-5spd/3.25 Trac-Loc 1968 Pontiac Firebird: Project Fire Chicken! 2015 Silverado Double Cab 5.3L Z71 2001 Jeep Wrangler Sport 4.0L 5spd 2020 Chevrolet Equinox Premium 2.0L Turbo 2011 Mustang V6 ~ Wife's ride American Born, Country by the Grace of God 1967 CST Shop Truck Rebuild! My 1967 C-10 Build Thread My Vintage Air A/C Install Project "On a Dime" Trying my hand at Home Renovation! 1965 Mustang Modifications!

02-16-2017, 03:06 PM	#5
67seaten Registered User Join Date: Nov 2016 Location: winnipeg Posts: 62	Re: rear end gear ration for t56 running a ls1 with a mild cam , gonna be alot of street driving some highway driving , i have a set 456's on the shelf for another project . if they do work well with this set up i will use them ,i didn't want to put it in they didn't work then end up taking it out __________________ mike

02-16-2017, 03:29 PM	#6
sangell Registered User Join Date: Jun 2015 Location: Overland Park KS Posts: 173	Re: rear end gear ration for t56 What is your t56 out of? They had different ratios, the most common behind the LS motors are the F-body version with a .50 6th. In my son's truck we have a CTSV T56 with the .56 6th gear and 27" tires. I put a 4.56 in it. With the low rear gear and high OD ratios you need to watch out for drive shaft RPM. His truck was a long bed and I tried to run a one piece shaft that didn't work out. With 27" tires, .56 6th gear it was 2400 rpm at 75mph which put the drive shaft at 4300 rpm and it was not happy. The drive shaft is a 4"x.083 wall steel piece. We have since cut the truck down to a shortbed so that makes the driveshaft 12" shorter and I'm dropping to either 4.10s or 3.73s. I think the 4.10s are the way to go but the 3.73s help so much with the drive shaft RPM. Last edited by sangell; 02-16-2017 at 03:35 PM.

02-16-2017, 05:42 PM	#9
Heavymetl Registered User Join Date: Feb 2012 Location: Southern Ohio Posts: 292	Re: rear end gear ration for t56 Driveshaft speed is the killer here. I'd say 4.10s would work, but look into aluminum for the driveshaft material. I run 4.30s in my T56 Camaro, but it's got a relatively short 3.5" diameter aluminum driveshaft and runs smooth as butter at any speed. __________________ 1970 C10 Turbo LM7 5.3/LS6 cam PT7675/4L80E/Holley Dominator EFI/E85 Moser 9" 3.25/4 wheel discs 11.16@121.07

02-16-2017, 06:58 PM	#10
sangell Registered User Join Date: Jun 2015 Location: Overland Park KS Posts: 173	Re: rear end gear ration for t56 When you get to big dia thin wall steel they have a equal or higher critical speed than an AL shaft does.

02-17-2017, 09:08 AM	#11
Mike C Registered User Join Date: Dec 1999 Location: Austin, TX, USA Posts: 7,714	Re: rear end gear ration for t56 Some good advice above. My recommendation for the .5 OD T56 would be 4.56 at a minimum. Especially if you are running a carburetor still. FI allows for even lower cruise speeds and good AF ratio. I wouldn't consider 4.88 to be out of line with a 30/31" tire either. __________________ 44 Willys MB 52 M38A1 64 Corvette Coupe 68 Camaro 'vert LT1 & TH700 69 Z/28 355 12.6's @110 69 Chevy Short Step 4 1/2"/7" drop 72 Jimmy 4WD 4spd 4" & 35's 02 GMC 2500HD 4x4 Duramax

02-17-2017, 09:50 AM	#12
sangell Registered User Join Date: Jun 2015 Location: Overland Park KS Posts: 173	Re: rear end gear ration for t56 This is from Mark Williams Attached Images

02-17-2017, 06:52 PM	#13
67seaten Registered User Join Date: Nov 2016 Location: winnipeg Posts: 62	Re: rear end gear ration for t56 thanks guys , its out of a camaro i am gonna stay efi , 27-29ish tall tire is what i am planning on running , aluminum drive shaft will be the way ill go , i will need a custom made once everything is together __________________ mike



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