Actual mileage advantage THR700 overdrive automatic?

[Wendell Hanks]

My '79 has a 3:07 differential and a GM Th400 short shaft transmission.

Have any of you replaced your TH (GM Turbomatic) 3-speed transmissions with a TH700R 4-speed to obtain increased gas mileage?

If so, was there any substantial difference in city driving compared to the previous trans?

My '79, with the replacement GM350 crate motor with 35,000 miles averages 15 mpg in the traffic-clogged Los Angeles basin.

If a replacement, I know Myers Studebaker has a "kit" that enables the replacement -- dealing with the new crossmember and the electronic substitutions.

Any insight and specific mileages are appreciated if you have made such a replacement or have an Avanti year that came with the TH700R 4-speed.

Thanks,

Wendell

[Gunslinger]

In my '70 I had the original Borg Warner 3-speed automatic replaced by a 700R4 along with the crate engine. I only have about 600 miles on the car since it was all done so I really don't have a handle on fuel economy yet...haven't even tried to figure the mileage at all, but it really does idle down the road at 70-80 mph. It has a 3.31 rear and can really smoke the tires if I'm not careful.

There was no serious issues at all for the speed shop that did the work...the 700R4 fit fine within the transmission tunnel and there was no problem locating the rear support. The only thing one could call a truly custom touch was they couldn't re-use the stock back-up switch and reverse light switch and had to fabricate new brackets due to clearance problems with the exhaust.

I had to get a new driveshaft so I went with an aluminum unit to reduce rotating mass. It wasn't much more expensive.

If you have the opportunity to install an overdrive tranny, you won't go wrong with either a 200R4 or a 700R4. The 200 should be a near drop-in since the length should be the same and you can re-use the driveshaft as is...unless there's something different about a short-shaft Hydramatic.

[theGman]

I appreciate your info on the conversion....I too am thinking of installing an overdrive automatic/injected engine in my '81. Does anyone know of a site which shows this being done in some detail?

Thanks,

Paul

[Gunslinger]

I appreciate your info on the conversion....I too am thinking of installing an overdrive automatic/injected engine in my '81. Does anyone know of a site which shows this being done in some detail?

Thanks,

Paul

I had Edelbrock fuel injection added to the GM Performance 350HO/330hp crate engine/700R4 installed in my car. The only site I know that shows it is on the Avanti Yahoo group where I have photos of my car being rebuilt. It shows the system on the bare frame and when the body is on. Don't know how much help that might be. Send me a PM if you have any questions and I might be able to answer them for you.

[Wendell Hanks]

Since my posting, I called one of the transmission companies called "Monster Transmissions" and spoke to the tech rep.

The outcome, based on the 3:07 differential, the 350 engine, and a 3500 lb. car, the advantage in the city (Los Angeles traffic!) estimate was "Maybe 2 mpg at the most."

Does this fit anyone's expectation who has installed the TH700R 4-speed?

Anyone with a post-1982 Avanti that has the factory installed 4-speed automatic and what is your mileage in the city?

Obviously, the cost will be at least $800+ for the trans and then the installation cost of $$$??. I know I won't be able to do this one in the driveway.

At 60 MPH, my Avanti is turning 2900 RPMs --- BIG difference with the Tacoma truck at 1800 RPM.

Many thanks.

Wendell

[Gunslinger]

I'm certainly no expert, but it seems reasonable that you would see more than a 2mpg increase with the transmission change. You could likely expect a significant reduction in rpm's at cruising speed and the engine would simply be loafing when not under load. That should result in much better fuel economy as well as reduced engine wear. I've talked to Corvette owners with a TH-350 or -400 that swapped to a 200R4 or 700R4 and many say they get upwards of 22-24 mpg after the swap...not all, as many still don't drive a Vette economically, but they still enjoy the lower rpm's at cruise and better fuel mileage, just not as much as some do.

It will take some period of time for the swap to pay for itself, but reducing engine wear and tear is benefit in itself.

[Twnchgr]

Since my posting, I called one of the transmission companies called "Monster Transmissions" and spoke to the tech rep.

The outcome, based on the 3:07 differential, the 350 engine, and a 3500 lb. car, the advantage in the city (Los Angeles traffic!) estimate was "Maybe 2 mpg at the most."

Does this fit anyone's expectation who has installed the TH700R 4-speed?

Anyone with a post-1982 Avanti that has the factory installed 4-speed automatic and what is your mileage in the city?

Obviously, the cost will be at least $800+ for the trans and then the installation cost of $$$??. I know I won't be able to do this one in the driveway.

At 60 MPH, my Avanti is turning 2900 RPMs --- BIG difference with the Tacoma truck at 1800 RPM.

Many thanks.

Wendell

Wendell,

I have a 88LSC with a 350cid 700R4. The ultra lo first gear will help your Avanti step out. Even with the 3.07 gears. Thou the .70 OD may lug it down a tad. Its going to think it has 2.14's in fourth gear. I can get 22-24 mpg if I keep it legal on the hiway, not very often.About 15-17mpg in town. converted from a 200R4 for about $1200.

Karl

[Wendell Hanks]

To all:

Thanks for the data.

The key point for me is the city driving -- I only use if for work not highway use. In LA, the perception is there is no "highway" driving, only stop 'n go.

Apparently, so far, the "Monster Trans" assessment was what twnchgr has experienced -- 2 mpg.

I'll post on the later model site when the 4-speed was standard and see if there is a pronounced difference in city use. However, I doubt that the cars are limited to that use with most drivers.

Now, with gas at 4.60 here, the initial $1200+ to change out the trans may be worth it.

Thanks for the input.

Wendell

[WayneC]

The key point for me is the city driving <snip>

Wendell

Karl... forgive my ignorance, but why did you change from a 200R4 to a 700R4?

Wendell... I've been told the best thing for mileage in city driving is to always keep the tach

under 2000 rpm, even on acceleration. Might help to shift to neutral at long signals, also.

Or, you could install an LT1 or LS1 (for fuel injection) along with a matching automatic :>)

[MikeValent]

You might get an extra 2mpg in city driving, but since most of the time you'll be idling, accelerating or decelerating, there's not much mileage influence a transmission swap can have for a carbureted v-8 engine in a 3,500 pound car.

[Twnchgr]

Karl... forgive my ignorance, but why did you change from a 200R4 to a 700R4?

If I knew then what I know now. Would still have a 200r4. Didnt have alot of info back then on the 200r4. And knew of several 700R4's that were being used in HiPo cars.But we broke Three before getting it right. Car leaves unbelievably hard with the lower 1st gear. The big drop into second is well covered by the Nitrous.In hind sight the 200 would have been a better choice.But I tossed the 305 for a 350. Now looking for a 3" crank to make a 302 out of it. Always doing something. Got a NACA duct to get more fresh air in the car today.Will mount it above the the parking lights.

[SBCA96]

Apparently the 200-4R has a little better gear spread, which makes sense since it

was designed as a small engine transmission. The 700-R4 was designed for larger

engines, and has a wider spread. The problem with most 200's is that it wont hold

up to the abuse of a even slightly modded V8 when stock. Weight is also almost

the same between the 200 and 700 so dont get fooled by that.

I found that general daily driving was improved with the low first gear, the car

felt like it was considerably faster. I would get mid-20's on the highway with my

modded Studebaker V8. You dont need the Myer kit if you have a Chevy V8. Do

make sure that you stick to the Gen 1 and Gen 2 bolt pattern trans though. It is

in your best interest anyway, as all Gen 3 are the electrically controlled, which will

be even better, but more expensive to set up.

Tom

[profaqualung]

SBCA96 [TOM] -

I have a reworked 200R4 in my El Camino - 450+ horse, turbo 383 SBC. It has close to 35,000 miles with this combo, and NO problems. I use it to pull my jetboat, around town cruising. Mileage is in the upper 23's on the road, as opposed to 19-20 with the standard 200 trans.

This trans was done almost 4 years ago, since then, a number of companies have been redoing the 200R4 to handle more power and selling them thru various Chevy restoration companies. ORIGINAL PARTS GROUP has a 200R4 for around $1,500 that will take over 400 horses. Internally, it is basically the same as the mods that were made to my trans - but with newer/improved parts. And I have yet to hear of anybody in the El Camino/Monte Carlo groups that have had any problems with them.

I feel sure there would be a limit on how much power these trannies will handle, but OPG should be able to help with any questions along those lines.

Now, that being said, my boat is hooked to the El Camino, and I am going to the Pearl River to watch the fireworks, and camp out for the rest of the weekend.

Everybody have a safe FOURTH.

Thomas

[SBCA96]

You can build anything to handle what you want, doesnt mean that one should go

and buy a used 200 from a salvage yard and expect it to handle a V8 engine. My

point is, its a dis-service to people to compare their 1500+ dollar special built trans

to a common everyday 200. The 700 can be pulled from a wrecked Camaro and

used right away - I wouldnt do the same with a 200.

Unless you pick up a 200 from a Grand National, stick to a '88 or newer 700R4.

Tom

I have a reworked 200R4 in my El Camino - 450+ horse, turbo 383 SBC.