Rear end ratio for 700R4?

[MiraclePieCo]

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.

[DransportGarage]

Originally Posted by MiraclePieCo

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.

[mongocanfly]

yep...2, 73 way to high for od...I would want around 3.73s

[G&R's57GMC]

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.

[_Ogre]

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

[DransportGarage]

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.

[whitedog76]

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.

[dsraven]

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.

[DransportGarage]

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.

[MiraclePieCo]

Originally Posted by DransportGarage

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.

[DransportGarage]

Originally Posted by MiraclePieCo

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.

[_Ogre]

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

[dsraven]

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

[Jesse Z]

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!

[mongocanfly]

yeah I would have changed rear gearing instead of ditching the 700

[jams97355]

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.

[roger55]

Originally Posted by _Ogre

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.

[bosco]

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.

[_Ogre]

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

[1project2many]

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.

... 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.

[roger55]

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.

[1project2many]

I'm sure we'll see a bunch of "no problem" responses, and that's fine. There are folks here who have good luck putting a V6 T5 behind an engine producing 350 ft/lbs or more as well. Hell, I used a 7.5" GM rear behind a 455 Buick in a Monte for 9 years without blowing it up. But that doesn't make any of these parts the best choice and it doesn't mean the parts are inherently good. The fact that there are survivor stories often says more about the user and the application than the part.

What's most frustrating to see in this thread is the number of responses suggesting a less expensive but practical solution involving a 2.73 rear and 350 trans should be dumped for a more expensive solution: buy new gears or rear axle then spend quite a bit more to rebuild and upgrade a transmission to go with the new rear. Why spend more to achieve what is easily done with less?

The Monte I mentioned had 2.26:1 rear gears and produced around 400 ft/lbs at 1600 rpm where it liked to cruise. Even though it only had three speeds (thm 400) it could take off plenty quick and still achieved 16-18 mpg on long cruises. I still have the car 29 years after I built it and I hope to get time rebuild it some day. I suppose I'd be told that I should ditch the rear for a 3.42 then spend money building and upgrading a 2004R if I asked for suggestions on the rebuild.

[DransportGarage]

Originally Posted by 1project2many

<snip>

What's most frustrating to see in this thread is the number of responses suggesting a less expensive but practical solution involving a 2.73 rear and 350 trans should be dumped for a more expensive solution: buy new gears or rear axle then spend quite a bit more to rebuild and upgrade a transmission to go with the new rear. Why spend more to achieve what is easily done with less?

<snip>

I suppose I'd be told that I should ditch the rear for a 3.42 then spend money building and upgrading a 2004R if I asked for suggestions on the rebuild.

Oops. I didn't realize that was what I was doing. I thought a member posted a question and I was helping him out -- sharing my experience. I thought that's what we did here, along with trying to find best (not necessarily cheapest) solutions for our trucks. At the end of the day I was actually going to allow the OP to continue to own his truck and make his own decision. Signing off of this thread...

[1project2many]

Dransport, you are very helpful and provide earnest advice as do so many here. This is a good community and signal to noise ratio is consistently good.

But please look carefully. Technically when the OP mentioned the THM350 he (or she?) did not ask a question. He posted a plan:

"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. "

Although not a common approach these days, I believe this solution has merit. OP posted benefits from following this plan and I believe they are valid. In fact I've tried the same approach. There are no gross flaws to making it work and no obvious reasons to say "bad idea." There are low ratio gearsets available for the 350 which can help improve acceleration off the line if it's a concern, although no concern has been raised yet. So I see no reason to push toward the 700 / numerically higher rear axle ratio if the OP is not so inclined.

I realize that you were just sharing an opinion. It was presented well and invited consideration. And ultimately either approach can net satisfactory results. But what followed your opinion was frustrating to watch. The wagon train started rolling without anyone directly addressing the usefulness of the OP's plan. I'm not sure why the thread turned into a defense of the 700R4 or of the choice to use one with a numerically lower gear ratio. But the internet is a fickle place and these things happen.

[evilokc]

stock gearing on my 1991 firebird is 2.73 with a 700r4. runs about 1700rpm at 70mph on the highway. smooth as silk. not a hotrod but it feels great in the highway.