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Old 03-07-2018, 02:42 AM   #1
MiraclePieCo
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Rear end ratio for 700R4?

Just decoded the GM 12-bolt rear that came with my '51 panel truck - it claims to be a 2.73 ratio. Is this too high to be suitable for a 700R4 overdrive trans? Stock SBC motor and trans, 28" tires. Most of my driving is freeway and I virtually always accelerate like your granny.

PS: Online speed/rpm calculators show that at 65 mph I'll be turning about 1500 rpm.

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Old 03-07-2018, 07:13 AM   #2
DransportGarage
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Re: Rear end ratio for 700R4?

Quote:
Originally Posted by MiraclePieCo View Post
Just decoded the GM 12-bolt rear that came with my '51 panel truck - it claims to be a 2.73 ratio. Is this too high to be suitable for a 700R4 overdrive trans? Stock SBC motor and trans, 28" tires. Most of my driving is freeway and I virtually always accelerate like your granny.

PS: Online speed/rpm calculators show that at 65 mph I'll be turning about 1500 rpm.
Just my opinion, but I say too high (unless you're setting up for the standing mile, in which case your theoretical speed at 6000 RPM will be 260 MPH).

**My granny thankfully drove Ramblers for the most part, but she thought the accelerator pedal -- and the brake pedal -- were on/off switches. (The phrase "stab it and steer" comes to mind.) True story: In a moment of brilliance, her son (my uncle) bought her an uber high mileage puke-green 4-bbl 350 '69 Nova when she was about 85 because it was cheap. Within a week she took out the windshield, the A, B and C pillars on the passenger side and the back window when she clipped the tailgate of a large truck. It was spectacular, and no one was hurt. I still remember her exclaiming "Ach du Lieber!" (OMG) as she described the event. Bless her heart and rest her soul.
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Old 03-07-2018, 08:56 AM   #3
mongocanfly
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Re: Rear end ratio for 700R4?

yep...2, 73 way to high for od...I would want around 3.73s
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Old 03-07-2018, 12:43 PM   #4
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Re: Rear end ratio for 700R4?

Yah the 2.75 is waaaay to high making 4th come in at 65 mph and then at 1500 rpm. Talk about gear hunting. 4-3 3-4-4-3 at light throttle

If you are using a 28" tire 4.11 would work great or maybe a 3.90 but nothing lower.

This would give great acceleration in all gears and decent power in 4th with a low rpm.

Remember that the gear spread in the 700R4 is about 1000 rpm between gears .
Use the tightest converter you can get and make sure it locks up at the lowest speed possible.
Ask me how I know.
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Old 03-07-2018, 01:13 PM   #5
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Re: Rear end ratio for 700R4?

depends on what you set your truck up for.
58 truk is setup for hiway cruising has 700r4 with 3.27 ratio, with the ramjet 350 it gets 20 mpg hiway
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Old 03-07-2018, 03:55 PM   #6
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Re: Rear end ratio for 700R4?

A fair number of rear ends have been re-geared. Before you contemplate a change, you may want to actually do a rough check on the ratio rather than go by the markings. There are tons of YouTube videos on how to do this, and some of them are close to correct! You can also check to see if you have a limited slip diff as well. If you have questions after you do this, ask away.
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Old 03-07-2018, 04:25 PM   #7
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Re: Rear end ratio for 700R4?

I'm thinking you need lower gears, especially if you have an early 700R4.

From experience, my 85 K10, with 3.42 gears had a hard time staying in OD below about 70.

A shift kit and TV valve made a world of difference, but was still kind of sluggish. On 2 lane roads I would hold it in 3rd, would shift in OD on interstate.

The 88 and later had the auxillary valve bodies, from what I understand, they were much better.
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Old 03-07-2018, 04:43 PM   #8
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Re: Rear end ratio for 700R4?

yup, I remember those earlier units couldn't decide what gearthey wanted to be in. shift up, shift down, shift lock up, shift out of lock up. rear axle ratio may make a big difference. I have always been a 3.73 ratio guy personally. my avalanche has 4.10's but that thing weighs a ton.
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Old 03-07-2018, 09:16 PM   #9
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Re: Rear end ratio for 700R4?

FWIW, my 700R4 55 2nd series has a 4.10 behind it (mainly because I found the rear end for $80 in a swap meet). Someday I will actually report back when I finally drive it, hopefully in this life.
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Old 03-08-2018, 02:41 AM   #10
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Re: Rear end ratio for 700R4?

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Originally Posted by DransportGarage View Post
A fair number of rear ends have been re-geared. Before you contemplate a change, you may want to actually do a rough check on the ratio rather than go by the markings.
Way ahead of you DransportGarage - I did a roll test today. Yep, it's an actual 2.73. So I'll be selling my 700R4 and replace it with a Turbo 350 - cheaper and simpler too.
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Old 03-08-2018, 03:16 AM   #11
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Re: Rear end ratio for 700R4?

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Originally Posted by MiraclePieCo View Post
Way ahead of you DransportGarage - I did a roll test today. Yep, it's an actual 2.73. So I'll be selling my 700R4 and replace it with a Turbo 350 - cheaper and simpler too.
Wow. I didn't see the switch to the TH350 coming. I would have been inclined to swap rear end gears - or even put up with the granny 2.73 - and keep the 700-R4 to take advantage of the overdrive.
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Old 03-08-2018, 03:26 PM   #12
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Re: Rear end ratio for 700R4?

the 700r4 has a lower 1st gear that will actually help you take off quicker than the th350

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Old 03-08-2018, 04:42 PM   #13
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Re: Rear end ratio for 700R4?

I was just gonna comment on that same fact. check the gear ratios between the trans choices because you may be surprised. maybe get you out of the hole quicker and easier on the torque converter which will build less heat which may extend the trans life etc. may make it more fun to drive too.
might be good to talk to a trans guy and see what he can do for you.
here is a write upon the 700r4 or the 4l60e. worth a read maybe?

http://www.hcdmag.com/700r4-transmission/

and another explanation with torque values etc

http://700r4.com/faq/ratiocmp.shtml

and another explanation

http://www.700r4transmissionhq.com/

here is one that shows how to ID the different units and a basic breakdown of the uses

http://www.700r4l60e.com/transmission_gm.html

hope that helps
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Old 03-08-2018, 06:40 PM   #14
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Re: Rear end ratio for 700R4?

From Bowtie Overdrive:

Solution
Install a TH-700R4 with these existing 3.08 rear end gears. This car will launch like a TH-350 or TH-400 with 3.73:1 rear end ears, so you can now have your stop light fun. When the Th-700R4 goes into fourth gear, your final drive ratio will be 2.16:1. (3.08:1 rear end ratio x .70:1 4th gear ratio in the Th-700R4 = 2.16:1 overall final drive ratio) With a 26" diameter tire, a 3.08:1 rear end ratio and the Th-700R4 in .70:1 fourth gear, your engine would be turning only 1670 RPM at 60 MPH. This is a pretty picture!
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Old 03-08-2018, 08:11 PM   #15
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Re: Rear end ratio for 700R4?

yeah I would have changed rear gearing instead of ditching the 700
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Old 03-10-2018, 01:44 PM   #16
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Re: Rear end ratio for 700R4?

I too am ditching the 700r4. Not for ratio reasons but I've already gone through 2 of them. A little over 500 HP and the tranny's wont hold up. Going to a th400. It's built with quality parts and should hold up better. Also running 3.08 rear so it should go down the freeway fine.
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Old 03-10-2018, 02:47 PM   #17
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Re: Rear end ratio for 700R4?

200r4 and 700r4 can be built to handle 1200 hp just like a th400 that is built right
in stock configuration neither will hold up to the abuse, any automatic needs to be built to handle the power
my ramjet is 350hp & 400ftlb torque i the paid money for a 700r4 that will last
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Old 03-10-2018, 03:29 PM   #18
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Re: Rear end ratio for 700R4?

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I too am ditching the 700r4. Not for ratio reasons but I've already gone through 2 of them. A little over 500 HP and the tranny's wont hold up. Going to a th400. It's built with quality parts and should hold up better. Also running 3.08 rear so it should go down the freeway fine.
X2 what Ogre said. There's a difference between high quality stock parts and high performance parts. Do you have any specific info as to what failed in your 700-R4, what upgrades it had, and what year trans it was? Just trying to learn more about these things...
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Old 03-10-2018, 06:14 PM   #19
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Re: Rear end ratio for 700R4?

I'm with ogre and dransport, build the trans to overfit the hp and torque being applied. also, the reason some "second time around" rebuilt units fail is because the cooling system for the trans was re-used without being cleaned properly. a multiplate cooler is VERY hard to get the debris from an exploded trans to purge out. some will install an inline filter but, personally, all that stuff goes away and is replaced with new stuff. rad, aux cooler, whatever. metal lines are cleaned with a solvent pump and then blown out with a "purge/cleaning plug" (looks like an expandable ear plug really) and compressed air, then brake clean and compressed air. if spending the cash for a new trans I don't want to run the chance of any of the old contaminants getting into the new trans through the cooler circuit. happened to me once, not again.
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Old 03-10-2018, 09:41 PM   #20
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Re: Rear end ratio for 700R4?

The 700R4 in my daily driver has 450,000 miles without ANY repairs of any kind. It get heavy towing use in a mountainous area, including a 28-foot travel trailer and frequent trips with the loaded car trailer. Serviced it once in that time. Just a V6 of course, but longevity of 700s is certainly beyond reproach.
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Old 03-10-2018, 11:28 PM   #21
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Re: Rear end ratio for 700R4?

The 2.73 will will work but it's higher than I would want in a truck. Good with a car and an engine with lots of low end torque though.

I've got a 3.54 in my '57 3200 with a 700R4 tranny and a 305 TPI engine. Perfect combo in my opinion. It will smoke the tires off the line and cruise nicely on the highway.
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Old 03-11-2018, 06:25 PM   #22
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Re: Rear end ratio for 700R4?

Although there are some very practical suggestions in this thread, I see valid reasons for keeping the 2.73 rear and ditching the 700. The THM 350 is cheaper to overhaul than the 700. The 700 often requires a number of upgrades just to overcome original design issues without even considering what it needs to survive a high torque environment. The 350 has less rotating mass which has the potential to save fuel in town. THM 350's are not popular and when they show up Craigslist in this area they can be had for under $100. The THM 350 is more likely to survive with a weak cooler and is less likely to fail if the detent cable is misadjusted. And although the 350 only has a 2.52:1 first gear ratio vs the 3.06 of the 700, the engine is not required to have as wide a torque curve to provide steady acceleration during the 50% drop in rpm at the 1-2 shift.

I agree with roger55. The question about the final drive ratio really depends on the torque curve of the engine. If you can put cruise rpm at the low to mid range of the torque curve you generally get better overall fuel economy. The Camaros and Corvettes equipped with TPI, six speeds, and numerically lower rear gears achieved very good economy and still had reasonable acceleration despite having mild power numbers. Tuned port engines had peak torque ratings of at least 330 ft/lbs with a broad torque curve providing at least 300 ft/lbs from roughly 1500 to 3500 rpm, plenty of torque for cruising.

Quote:
... but longevity of 700s is certainly beyond reproach.
Although your transmission is indeed remarkable, I will have to respectfully disagree with the conclusion. 30 years as a professional technician, with the majority of my time spent with GM vehicles, has brought me far too many prematurely failed 700R4 transmissions. The list of factory changes and "upgrades," most of which were designed to keep the transmissions from failing while in warranty, is huge. And the aftermarket has thrown no small amount of time and money into the fight as well. Yes, a 700R4 built today has a much better chance of surviving than an old factory unit. It would be nice to attribute this to excellent engineering, but truthfully it is because the aftermarket and GM combined have 36 years worth of trial and error trying to turn it into a decent transmission.
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Old 03-11-2018, 07:55 PM   #23
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Re: Rear end ratio for 700R4?

I've owned 4 vehicles (85,88,90,91) with 700R4 transmissions and 1 (96) with a 4L60e. Never had a single issue with any one of them. All stock engines though (4 TPI and 1 LT1).
I think it's a great transmission. Don't mind the 1-2 split at all. I'm not a racer though.
That said, I've always heard to stay away from early 80s versions.
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Old 03-11-2018, 08:31 PM   #24
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Re: Rear end ratio for 700R4?

Quote:
Originally Posted by _Ogre View Post
the 700r4 has a lower 1st gear that will actually help you take off quicker than the th350
Ogre, your chart isn't correct. A 700R4 has a .70 4th not a .75.
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Old 03-11-2018, 11:26 PM   #25
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Re: Rear end ratio for 700R4?

I have had 3 trucks with 700r4 with no issues. My 56 has 3.73 with the 700r4 and wouldn't ask for a better combo. Takes off great, but best of all cruises smooth on the highway. I'd go with this combo every time.
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