The Stovebolt Site Forums The Shop Area Driveline Rear gear ratio.

Sorry guys but need help again please. I have a 51 that has a oldsmobile 350 with a turbo 350 automatic tranny and it has a stock 1955 rear end. My cruising speed at about 65 mph is turning around 3100 rpm. It has the factory 3.90 gear ratio. If my calculations are right if I change over to 3.08 it will drop rpm's about 500 or 600. My question is can this be done. I would also like to change it to a posi-track. Is this possible or have I gone crazy. I need to get my rpm's down so I can cruise on the highway. Thanks for help everyone.

Does 3.08 come in '55 axles?

Ed

I have a '49 with a Chevrolet 350 and a TH400 (Same 1:1 final gear as your TH350) with a 3.08 rear end. It doesn't like to do burnouts but it does like to go 70.
Having said that, I wish I would have gone with a 700R4 for the lower first gear (for burnouts, man) and the overdrive for better gas mileage on the freeway.

I have about 435 hp so I don't think burnouts will be a problem. But glad to here about being able to do 70 down the freeway. I am not sure the 700r4 will fit because the bell housing for the turbo 350 is about a half inch from the floor of the truck.

The differential shop said they could put 3.08 in any differential from a 55 to 64.

I'd choose something in the 3.20-3.30 range, like maybe a 3.23:1. IMHO, 3.08 gears are too tall unless you're running very low profile tires. I ran a 58 Chevy coupe for a lot of years with a 3.36 gear, a 283 3 speed, and 14" wheels, and I had no trouble at all getting 70+ MPH speeding tickets.
Jerry

Herb,

It's easy to replace a TH350 with a 200 4R because they are the same length and the driveshaft does not have to be altered in any way. The 200 4R is more compact than the TH 400 making it easy to fit it where the TH 350 fit. When I did that conversion on my roadster street rod the changes I had to make were moving the rear mount rearward about 6 inches, getting a correct shifter and getting a TV cable and geometry correcter from Bow Tie Overdrives who also provided the transmission.

If you do some research on the 200 4R you'll find out what the enthusiast magazines have to say about its ability to handle various power levels.

The 0.67 OD ratio would be a nice match with your 3.90 rear end.

Good luck and have fun!

Thanks for the information yard that's what I want to do is go to a 200r4 so I can keep the 390 but use the overdrive for the highway. Does the 200r4 use any electronics to run it.

If it don't have a "E" in it, it ain't "E"lectronic. In other words, not computer controlled. I believe it does have 12V supplied lock and a TV. You are well advised to use a 200R4. Small size and good tranny. 1st gear is 2.7. Be aware of BOP bolt pattern. If 200r4 was used in those, it may not have a dual pattern. Just check that it fits a Chevy motor before you buy.
Stay with the rear ratio until you drive awhile with the 200r4.

Thanks everybody. I like my 3.90 so I will go for the 200r4.

Anyone know if there is any difference in external dimensions between a 200R4 and a 700R4? 700R4s are easier to find than the 200R4s.
I won't get into the reliability comparisons between the two, I just need dimension info. The issue is fitment of the transmission in relation to bottom of the cab of an AD.

Carl
resident thread absconder

i can tell you i replaced the TH350 with a 700R4 in my Camaro and I did need to change the crossmember and shorten the driveshaft. if the poster above is correct that a 200R4 directly replaces a TH350 then there is a difference between the 200R4 and 700R4. I had no other issues with the swap regarding dimensions (floor/tunnel interference, etc)

http://www.tciauto.com/tc/trans-dim/
Yar and TomTom are right on.

Looks like certain 350's and 200r4 (2004r) have the same O/A length, the mounting distance from bell is different.
200 and 700 are different in O/A and Mtg.

The web has many charts and dimensions. Many have different numbers than my link, many are the same. The transmissions are not real easy to measure accurately without a square and/or a plumb bob, level, etc. Many folks have not learned how to use a tape measure yet. They put bad info on the internet. They fail to say they eyeballed it. I figured that TCI would probably be the best source considering their reputation and the fact they focus on trannys. I wouldn't do any cutting or welding from internet dimensions myself. I have to have the actual part. The Stovebolt member's information is a great source as you can see. (non-absconders) Dead nuts correct.

Carl: "relation to bottom of cab" ? as in what relation? hump fit or stick location or..........

"Does the 200r4 use any electronics to run it."

Herb,

There is a 2-wire plug that goes into the left side of the transmission. I put my 200 4R trans in about 15 years ago so I don't remember the details of that electric connection but I think it is connected to the brake light switch to unlock the converter when the brakes are applied to keep from killing the engine if the rear wheels skid in a panic stop. I still have all the paperwork that came with the trans from Bow Tie Overdrives and I can look that up for you if you like. Just let me know.

"Anyone know if there is any difference in external dimensions between a 200R4 and a 700R4? 700R4s are easier to find than the 200R4s."

Carl,

Both the external dimensions and the gear ratios are different as I recall.

The issue is at the junction of the toe board and the floor board where it interferes with the hump on the top of the auto transmission. Some sheet metal needs to be tweaked to make the TH400 fit. I am wondering if that area of the 200R4 is the same as the TH400.