Highest Gear Ratio with TH400 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

cwilkie · 06-15-2005, 10:33 PM

What is the highest gear ratio that can be put in a rear end using a TH400? I would like to lower my RPMs a little. Currently I have a 396BB with a TH400 using a 3.07 rearend. Some would say to go to a 700r4 but I want to keep my TH400 because of sentimental value. Anyway, all tech advice is needed.

Chance

shortbed70 · 06-15-2005, 10:42 PM

Are you wanting to go with a smaller numerical gear? How tall of tire are you running? What rpms are you turning at what MPH?

Robert1970C20rstbukt · 06-16-2005, 12:10 AM

I always thought that 3.07's were as high as you could get?

67chevemall · 06-16-2005, 12:21 AM

ya youd probaly have to get a custom rear end.
That set up is great for long hauln and pullen though.
email the Nascar tech's if you want to go Fast.
Are ya buzzn for speed?

tomatocity · 06-16-2005, 12:41 AM

2.73's in a 1995 Camaro with a 4L60e auto. 2,000 rpm's at 77 mph. Cruisin'.

Quote:

Originally Posted by cwilkie

What is the highest gear ratio that can be put in a rear end using a TH400? I would like to lower my RPMs a little. Currently I have a 396BB with a TH400 using a 3.07 rearend. Some would say to go to a 700r4 but I want to keep my TH400 because of sentimental value. Anyway, all tech advice is needed.

Chance

cwilkie · 06-16-2005, 10:03 AM

I want to decrease my fuel consumption and I am buzzing for speed. Actually, I would mind getting a heavy duty 700R4 but it's not cost effective in my current situation; however, I found one I liked at Mad Dog Transmissions (The God Father) but I do not have that kind of ca$h at the moment.

I have mid 1980s Chevy truck 15x8 really wheels. I do not know how fast I am going since the speedo gear is wrong; however, I would guess at 2500rpm I am going 50-55mph.

Are 2.73 the next step higher to 3.07? Suggestions are more than welcome...they are needed.

Chance

**kwmech** · 06-16-2005, 10:31 AM

The 2.73 ratio fits a later model 10 bolt axle. Has a carrier all to itself.

67ChevyRedneck · 06-16-2005, 11:40 AM

Hate to say it, but the only way your gonna get what you really want is a 700R4, like what has already been said, you're not going to see a big drop in RPM's. Check out the link below. You input tire height, a speed, and rear-end ratio and it will give you rpm's for and auto tranny and an AOD tranny. If you input different rear ratios you will notice there isnt much of a drop from a 3.07 but the700R4 will give you a noticable drop. My 67 with a 350/700R4/3.73 runs about 2,400 rpm's at 75mph and my 2000 Z71 with 327/4L60/3.73 runs about 2,000 rpm at 75 mph. With a 700R4 and 3.07 I wouldn't recommend any heavy hauling, might want to swap to a 3.73 for that......

RPM calculator

lloyd k · 06-16-2005, 11:15 AM

i have a 2.73 ratio in my 67 converted to leave spring rear,it works well with my combo because i have a big block cadillac in it.my carrier has a 12 bolt cover not sure what year it came out of,i would assume a mid 80s truck,i changed from a 3.42 and it made a noticeble difference all the way around.not sure you will see a big difference from a 3.08.

67chevemall · 06-16-2005, 10:01 PM

Get a standard 1:1 4 gear tranny and play with the gears?

Green Machine · 06-16-2005, 11:56 PM

Check out link below, they seem to have a 2.78 gear that will fit the 3.07 carrier, that will drop your rpm about 200 rpm. Going from say a 28 inch tire to a 30 inch will drop another 150 rpm. New gears, setting them up and new rear tires are going to set you back serious bucks. I think it would take a LOOOOOOOONG time to pay that back.

http://www.oldchevytrucks.biz/catalog3/0044.html

06-15-2005, 10:33 PM	#1
cwilkie Chance Join Date: Sep 2001 Location: Belcourt ,ND, USA Posts: 750	Highest Gear Ratio with TH400 What is the highest gear ratio that can be put in a rear end using a TH400? I would like to lower my RPMs a little. Currently I have a 396BB with a TH400 using a 3.07 rearend. Some would say to go to a 700r4 but I want to keep my TH400 because of sentimental value. Anyway, all tech advice is needed. Chance __________________ 1968 Chevrolet Stepside BB 396 w/TH400 1972 Chevrolet 3/4 Ton 4x4 BB 496 w/TH400

06-15-2005, 10:42 PM	#2
shortbed70 Registered User Join Date: Mar 2002 Location: Forney TX Posts: 4,512	Are you wanting to go with a smaller numerical gear? How tall of tire are you running? What rpms are you turning at what MPH? __________________ Troy 1965 Chevy Bagged,361 sbc,voodoo cam,1.5 full roller rockers,patriot 185cc vortec heads 2.02-1.60,vortec weiand polished intake,demon carb my truckhttp://67-72chevytrucks.com/vboard/s...d.php?t=332884 Big Red Dog build http://67-72chevytrucks.com/vboard/s...d.php?t=572274

06-16-2005, 12:10 AM	#3
Robert1970C20rstbukt Registered User Join Date: Jun 2004 Location: Beeville, Texas Posts: 1,943	I always thought that 3.07's were as high as you could get? __________________ RJD 1984 Chevrolet C30 welding truck 454/SM465 1977 GMC C15 pickup 350/TH350 1970 Chevrolet C20 pickup 350/TH400 1970 Chevrolet K20 Suburban 454/TH350 1969 GMC 8500 478M V6/5 spd winch truck

06-16-2005, 10:03 AM	#6
cwilkie Chance Join Date: Sep 2001 Location: Belcourt ,ND, USA Posts: 750	Gears I want to decrease my fuel consumption and I am buzzing for speed. Actually, I would mind getting a heavy duty 700R4 but it's not cost effective in my current situation; however, I found one I liked at Mad Dog Transmissions (The God Father) but I do not have that kind of ca$h at the moment. I have mid 1980s Chevy truck 15x8 really wheels. I do not know how fast I am going since the speedo gear is wrong; however, I would guess at 2500rpm I am going 50-55mph. Are 2.73 the next step higher to 3.07? Suggestions are more than welcome...they are needed. Chance __________________ 1968 Chevrolet Stepside BB 396 w/TH400 1972 Chevrolet 3/4 Ton 4x4 BB 496 w/TH400 Last edited by cwilkie; 06-16-2005 at 10:08 AM.

06-16-2005, 11:40 AM	#8
67ChevyRedneck Hittin E-Z Street on Mud Tires Join Date: Nov 2004 Location: Greenville, SC Posts: 23,090	Hate to say it, but the only way your gonna get what you really want is a 700R4, like what has already been said, you're not going to see a big drop in RPM's. Check out the link below. You input tire height, a speed, and rear-end ratio and it will give you rpm's for and auto tranny and an AOD tranny. If you input different rear ratios you will notice there isnt much of a drop from a 3.07 but the700R4 will give you a noticable drop. My 67 with a 350/700R4/3.73 runs about 2,400 rpm's at 75mph and my 2000 Z71 with 327/4L60/3.73 runs about 2,000 rpm at 75 mph. With a 700R4 and 3.07 I wouldn't recommend any heavy hauling, might want to swap to a 3.73 for that...... RPM calculator __________________ 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!

06-16-2005, 12:21 AM	#4
67chevemall Registered User Join Date: Apr 2003 Location: NB Posts: 3,367	ya youd probaly have to get a custom rear end. That set up is great for long hauln and pullen though. email the Nascar tech's if you want to go Fast. Are ya buzzn for speed?

06-16-2005, 10:31 AM	#7
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,599	The 2.73 ratio fits a later model 10 bolt axle. Has a carrier all to itself.

06-16-2005, 11:15 AM	#9
lloyd k Registered User Join Date: Feb 2004 Location: columbus,ohio Posts: 201	i have a 2.73 ratio in my 67 converted to leave spring rear,it works well with my combo because i have a big block cadillac in it.my carrier has a 12 bolt cover not sure what year it came out of,i would assume a mid 80s truck,i changed from a 3.42 and it made a noticeble difference all the way around.not sure you will see a big difference from a 3.08.

06-16-2005, 10:01 PM	#10
67chevemall Registered User Join Date: Apr 2003 Location: NB Posts: 3,367	Get a standard 1:1 4 gear tranny and play with the gears?

06-16-2005, 11:56 PM	#11
Green Machine Senior Member Join Date: Jul 2004 Location: Estherville, Iowa Posts: 3,371	Check out link below, they seem to have a 2.78 gear that will fit the 3.07 carrier, that will drop your rpm about 200 rpm. Going from say a 28 inch tire to a 30 inch will drop another 150 rpm. New gears, setting them up and new rear tires are going to set you back serious bucks. I think it would take a LOOOOOOOONG time to pay that back. http://www.oldchevytrucks.biz/catalog3/0044.html __________________ 1968 C10 307 3spd Long Fleet ------ http://67-72chevytrucks.com/vboard/s...d.php?t=201103 1970 C10 305 Super T10 Long Fleet --- http://67-72chevytrucks.com/vboard/s...d.php?t=202285 1971 C20 383 TH350 Dana Posi ----- http://67-72chevytrucks.com/vboard/s...d.php?t=206894 2001 GMC Sierra 1500 C3 6.0



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