[JLMounce]

I had a set of 3.73's with 26" tall tires and on the highway I hated it. 3400 rpm at 65 mph for long stretches is just a bit hell on the driver. At least this driver anyway.

I opted for a set of 3.31's as a replacement behind the factory th400 and it's much nicer. It's kind of amazing what only about 400-600 rpm will do for your comfort. The 3.31 to 3.55 range also works well with a lot of OD transmissions, from the 4l80, to many of the manual offerings.

[67Fbird]

3.55 would be NICE and a WORLD of difference in the feel of the car. With that said, coming off of 2.56's you will need an OUTSIDE the transmission speedometer gear set to calibrate your speedo....or tear into the th400 and replace the drive gear (as you can only get so many combo's. OR...to hell with the speedo and drive by your tachometer Kinda useless to spend $200 on a speedo fix to only change out the tranny 1 yr later.

[Formulas]

I'm my experience with old school transmissions and 3.75 stroke street engines life starts with a 3.55 gear just not enough low end torque with any kind of cam

[android 211]

A lot of posters have said they loved their overdrive automatic transmissions. Its a lot of money and several modifications to the car to accomplish it. Have you considered a gear set change to your Turbo 400? There are 3:1 first gear and 2:1 second gear sets available to get you off the line and keep the easy cruising 2.56s in back. Its what, $600 plus the R&R of the trans? I went from 2.56s in my 69 GTO (mild 455) to 3.23s. Now I cruise at about 3000 rpm at 65-70. I might go back to the 2.56s. Most of my driving is around town but next year I hope to Power Tour it.

[Formulajones]

That works to a degree but it's still not reaching the full benefit the overdrives offer.

Agree overdrive swaps are not cheap to do correctly, but they can be done without modifying anything on the car that can't be turned back later. But I'm betting once done you'll have no interest in going back. I haven't met a single person yet that has regretted the change, despite the costs involved.

[track73]

My 79 Trans Am is a heavy car around 3900 lbs. With the engine and trans that's in my signature, the engine has so much low end torque that I can cruise in town in 4th gear with no problem. Going down the highway at 71 is more enjoyable.at 2500 rpm.

[Steve C.]

Consider the type of torque converter in use, a non lock-up or a lock up type.
Art Carr does not believe in using a lock-up torque converter in any aftermarket-engineered, high-performance application. Mine is such, I have 505 cubic inches with over 600+ HP. Personally with my overdrive transmission I use a non lock-up converter built by Continental Torque converters. Out on the open highway at a steady cruise speed it has about a 200 rpm slippage. When figuring in your rear gears consider this topic.


.

[Formulabruce]

One thing of note, you can get your O/D built to your engines Torque.
Cheapest would be a bolt in 2004r. Has a .67 overdrive.
With lock up, can really help with economy, drive ability.
GM used 3.42 with Overdrives as standard in the 90's, 2000's .
The highest you want is a 3.08 If cruising, the 3.08 can really increase your range.
I run a 3.42 with a stage 3, 2004r and lock up, stock rpm converter. Can do spirited driving, and get 80 mph at 1990 rpm.
PAID FOR ITSELF IN 3 YEARS with fuel cost savings.
Can do s

[Formulajones]

I went the lockup converter route on my 4L80E.

I did a custom 9.5" triple disc that can be locked under power which is fun to experiment with.

[Steve C.]

Example only, not any endorsement nor recommendation.

With my 200-4R transmission. Under a steady state cruise speed on the open highway with about 200 rpm converter slippage, 27.7" diameter rear tire and 3.73 gears it is about 2300 rpm at 70 mph.


.

[76TA462]

Summer 2020 I installed a 2004R (Extreme Automatics Stage 2). I stayed with the original 3:23 gears, and have 27 inch tires. Works perfect for me and highway driving is a dream compared with the TH400. I am sure a 3.42 or 3.55 would be fine too, but I stayed with the original rear for convenience (and money towards other tweaks). Should I toast the 3.23 down the road, there is a good chance I will stay with the same ratio.

[David Jones]

My 2016 F150 2.7 with the 3:31 gears and and the 6spd cruises at around 1800rpm at my normal 72-73mph.
My GTO with the .64 TKO and a 3:73 is at about 1900+- doing the same.

[Formulajones]

I really like the TKO's with the .64 overdrives. Our daily with 3.55's and a 27" tire cruises nice at 2000 rpm and 70 mph getting just over 21 mpg with a fat carb on it. We've even buzzed 85-ish mph when in a bit of a hurry and I still knocked down 20 mpg with it on that trip. It just doesn't seem to care.

Overdrives are a game changer.

[JLBIII]

Originally Posted by 67Fbird

OR...to hell with the speedo and drive by your tachometer Kinda useless to spend $200 on a speedo fix to only change out the tranny 1 yr later.

Already doing that since speedo doesn't work. LOL Also use GPS for mph.

Lots of good input here. Any advantage the keeping TH400 and going with Gear Vendors OD vs replacing it with an OD trans. Car still has original engine and trans if that makes a difference.

[geeteeohguy]

Originally Posted by JLBIII

Currently have 2.56 gears and Saf-T-Trak rear un my 68 FB with 400HO and TH 400. I'm looking at replacing gears and carrier to either 3.36 or 3.55 and the correct posi unit. Plan to eventually put in an OD. Which would be the better street gears for the 400HO engine? Thanks

My '67 GTO convertible with a standard 400 and TH400 trans came with a 3.36 rear gear and it was decent all-around. I do a lot of long distance high speed driving, so about 15 years ago swapped in a 2.56 rear that had safe-t-track. With the 3.36, I got about 15-16 mpg at a steady 65 mph and the comfortable cruise was about 70 mph. The 2.56 gives me 21 mpg at about 80 mph, and 23 mpg at 65. 3000 rpm is a tick over 90 mph. So for me, the 2.56 was an 'upgrade'. Not as strong off the line, but still burns two tires. Ideally, for me, an A or F body with a TH400 would be perfect with a 2.93-3.08-3.23 rear gear. Pontiacs don't need a steep gear to get going like small block Chevys do. It all depends on the type of driving you do. If I just did short trips on back roads, I wouldn't of changed the 3.36 out. But I do cross-country trips and enjoy the increased MPG, decreased running temps, and almost no noise at high speeds.

[Formulajones]

That's going to be more individual preference JLBlll.

Gear vendors work pretty well and are much easier to install, and end up being a little cheaper as well. They only offer about a .78 overdrive but if you only run a 3.23 to 3.55 gear, it works out pretty good. You also don't get the benefit of a lockup converter, so if you have any kind of stall converter in that 400 at all, it really works best if it's a well built converter that couples well at higher speeds. Similar to what Steve mentioned earlier, how his non lock up converter only slips about 200 rpm or so. That's a pretty well built converter that couples well and won't produce a ton of heat at low rpm overdrive style cruising.

Depends on if you're really dead set on keeping the original trans in the car. Numbers bother some people. For me, I feel there is no harm in removing it for something better as long as the original is kept, and the mods are reversible.

[geeteeohguy]

Originally Posted by 67Fbird

3.55 would be NICE and a WORLD of difference in the feel of the car. With that said, coming off of 2.56's you will need an OUTSIDE the transmission speedometer gear set to calibrate your speedo....or tear into the th400 and replace the drive gear (as you can only get so many combo's. OR...to hell with the speedo and drive by your tachometer Kinda useless to spend $200 on a speedo fix to only change out the tranny 1 yr later.

I installed a different driven gear unit that bolted right up in a few minutes when I went from 3.36 to 2.56. At the time, the whole set-up was under $30. Speedo is right on the money, and I saved my old unit in case I ever re-install the original unit (doubtful).

[78w72]

with all the mentions of 200r's on this thread & quite a few guys saying they run one... have any of you had any problems or reliability issues behind mild built 400/455s? i know the 200r can be built to handle a lot of power, look at the fast buick GN guys & most are still using the 200r, but i read on here recently from one guy that said they are absolute junk & will blow up if you look at them wrong...

[Formulajones]

I think what 67bird is referring to is the small range of drive and driven gears available for the 400 turbo.

Generally when running a 3.73-4.10 ratio, or the 4.33's, the 400 needs a 15 tooth drive gear and only the 40-45 driven gears will service those rearend ratios.

When you start dropping the rear gear down to 3.55's and lower, the drive gear needs to be swapped to a 17 tooth because there isn't enough range on the driven gears to correct it. And that involves removing the tail shaft and pressing that gear on and off the output shaft. Not exactly a fun process, not terrible, but not fun. Then to get into the 35-39 range driven gears, the driven gear housing now also has to be changed. 400's are just kind of goofy that way. Not always fun to dial in a speedo with a 400 turbo and actually starts to get a little spendy these days.

You can monkey with tire heights to fiddle a few mph up and down as well but it seems most people are set on a certain wheel and tire package that won't allow that.

[Johnny406]

If you have or can pick up a 12-bolt, a 3.31 would be a wonderful gear for you.