I am wondering if it is worth putting an overdrive in the chevelle I have a mild 350 and I already own a 1991 4l60e tranny out of an s10 with a 4.3 that has been rebuilt but I never used. What kind of mile could I expect.

I have this setup in a 69 Camaro that I restored last summer. It has a 700r4 with a 3.55 posi (with mild 350). I have never checked the milage but can confirm that it is pretty good-much better than my Chevelle (454 with TH400 and 3.73). It is a joy to drive the Camaro on the highway where it cruises easily at 75mph. The Chevelle would be wound tight if you did that. I am going to a 3.08 gear in the Chevelle to get it more drivable. At age 57 I am now a cruiser and not a dragster.
Mike

With the 4L60 E is computer controlled so you will need a brain box from GM to run that trans. A regular 700R4 or 4L60 that is not would be a better choice.

I have a 700 R 4 In a 70 SS 454 Sta. Wgn. I know its not a small block but I had to get my 2 cents in. With a 396 in this car I was getting 22-23 at 75 mph engine was turning 1950rpms. Stuck in a 454 same drive line 3:08 12 bolt. Now on the highway with 3:08 rear, 19-20 mpg at 75 mph. engine turns 1950 rpms And turns low 15s at the strip. You will love a 700. GO FOR IT!!!
Bob ACES 2825

Same here! 396, 625 CFM carb, mild motor, 3:55 gears and 15 inch wheels and my beloved 700R-4 gets about 16.5 MPG and runs the 1/4 in 13.50 - 14.00 depending on how cold it is. My Big Block just loves cold weather. 40's to 50's...........

Straight 4L60 is just a renamed 700R4.
4L60-E came out in 1993 in trucks, SUV's, vans, etc, didn't appear in passenger cars like the Camaro/Corvette until 1994 model year. You WILL need a controller to run a 4L60-E, but will NOT need one to run a 4L60/700R4. The only thing with the 700R4/4L60 that needs electronics is the lockup converter, plans to build a controller are all over the internet including this site, or you can buy one pre-made, or control the TCC hydraulically.

Personally, I wouldn't bother with the 4L60-E, its too much $ and hassel to get one to work in an older vehicle. Not that you can't, you certainly can, but nowadays the 4L80E has come way down, and for ALMOST the same effort, and admittedly, a little more $, you can put one of those in, and never, ever have to worry about breaking it. JMO.

Back to your mild 350 setup: 700R4's or 200-4R's are much easier to install than the electronic versions, and for a mild 350 will do just fine with minor (and very common) mods.

BTW a 4.3L bolt pattern WILL fit a V8, its the 60 deg family of engines (2.8, 3.1, 3.4, etc) that have oddball trannies. However, I am pretty sure a 4.3L tranny will have V6 internals, smaller shafts, less clutches, etc. Ask a tranny guru to make sure, but you don't want a V6 core in there. Even a mild 350 will eventually tear it up.

Also on the off-chance you are referring to a 4wd truck the output shaft would be shorter, and the extension housing missing, which would have to be converted for a passenger car application.

You WILL see an ENORMOUS improvement in gas mileage. It IS worth the time and trouble. Overdrive is the only reason I am running a 4.10 rear cog. With the lockup convertor it gets 20mpg highway, although much less if you're thrashing it. But long distance cruising is now possible, comfortable, quiet, and reasonably affordable.

I was checking the RPM calculator and according to that with my 2.73s and the 2004r I should be running 1700 RPM at 75, and 1200 at 55. That's almost idle speed! Dan if you can get 20 with 4.10s I spect I should get quite a bit more than that. I'm gonna sacrifice a little bit of that mileage for some neck snap, me thinks. :)

20 mpg is only on long cruises, with the eggshell under-the-foot method. Every motor is different, too of course.

But I figure our cars are never going to be econo-boxes, so I'd swap mpg-for-neck-snap any day of the week. Neck-snap was first and to get a reasonable mpg I went overdrive. A 2800rpm lockup converter also helps.

I get better mileage in my Chevelle than I get in my 88 S15 Jimmy, but only on the highway. Doesn't take a genius to guess which one I'd rather drive, though :)

They do tell you to try and run on the idle circuits at cruise, if possible. I don't know how happy a car would be to lug along in top gear at 1200 rpm, I would think a motor would prefer 1500-1800 rpm, and get just as good mileage, but you never know.

i put a 700r4 out of a 92 Caprice in my Monte last summer, and my mileage went down- except for thoe 100 mile jaunts down the interstate, then it stayed the same as the TH350- 17 mpg or so. if i had paid money for that trans, i'd be disappointed, but i had it and the lockup kit lying around, so i threw it in.
of course, i've got the mighty 140 hp 350 that came stock in the car, 2.73 gears, and 275/60/15 tires on it.. the engine just kind of lugs along at speeds below about 70mph. takeoff feels stronger, tho, due to the deeper frst gear in the trans compared to the TH350.
i want to put some deeper gears in it- like 3.73 or so- and upgrade the cam to a more torque oriented aftermarket piece someday. i'm sure that would help a bit.
so, if you have highway gears, and low torque output, you might actually see a loss of economy for all your $$$ and effort.

140 hp 350, thats the L-65, right? LOL, I had a few of those.

Not enough torque to pull overdrive! Good point.

Hopefully "mild" is different than "sedate".

i'm getting 20ish with mine

mild 350 (268 cam, holley spreadbore, 3.08 rear)

I'm kinda wondering if the carb might be due for some work, its been real moody. I can skrew the idle mixture skrew all the way in on either side and it won't begin to die till the other is within 3/4 turn from being all in too, i dunno if this is normal