I have a 1964 F85 with a mild 425 I built in 2020. I have had a TH400 built and has sitting in my garage since covid that I haven’t swapped in yet. I was waiting on a ford 9in but they were back ordered at the time from speedway during covid. Had a couple kids and haven’t touched it since lol.

Looking to finish it. I’m running a 255/60r17 rear tire so 29in tall tire. Obviously drives down the highway great with 2.78 gears. I haven’t bought a converter yet. Guessing 2800-3000stall? Cam is .496/.504 230/234 @.050 not sure what rear end ratio to go with. 3.00, 3.25, or 3.50? I have a 69 Big Block Dart with 3.73’s and that screams down the highway. So don’t really want to go that high. Pros and cons between the 3 options I listed?

 

Back in the day, the factory considered 3.23 to be a good compromise between off-the-line power and low cruising rpm for a TH400.

 

I don't know what you call "screams" but with 29" tall tires and 3.73 gears, your engine will be turning about 3100 RPM at 70 MPH. That's nothing.

 

Back in the day as a far more virile yout, I commuted weekly between Providence and Hartford in a 65 442 with 4.11s, M20 and not large diameter tires, that definitely sang a bit. Later, my daily driver was an LS6 with 4.56, M22 & short tires, admittedly I travelled freeway routes only if necessary, but it was great around town! Didn’t hurt the cars any, a little tough on the wallet when my trusty steeds had a Sunoco 260 drinking problem.

In my now advanced age, I love my 3.90/200-4R combo but that’s kind of having cake and eating too. I also like the other car with 3.08/200-4R combo. My elCo runs a 3.36/TH350 and it’s a good all around combo.

In my observation, anything lower than 3.5x tends to freak out “Average Joe” pleasure driver unless they have an OD. In common Olds rears 3.23 - 3.42 seem like good compromises. Not sure what is available for 9” but I would guess that range would provide a nice boost without fretting on the freeway.

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If you haven’t bought the 9” from speedway yet, may I suggest a QUICK Performance rear? They’re usually a bit cheaper. My 9” from QP is stought.

i really liked my 3.42’s when I ran my 10 bolt.
Now I also like my 3.55’s…with an M-21.
My 3.90’s were great with a Gear Vendor OD…but now it just goes around town and to the track with just a TH400.

 

For sure. I have not bought anything yet. I forgot about QP. They're also not to far from me. But, neither is speedway. I'm in Western Iowa. I have heard good things about QP as well.

 

What rpm are you comfortable with? The online rpm calculators don't factor in torque converter slippage, add 300 rpm minimum for a higher stall especially. The 29" tall tire will drop about 200 rpm on the highway vs the factory tire height. With the 3.25 gear and 29" tall tire, around 3000 rpm at 70 mph. The 3.55 adds about 150 rpm at 70 mph.

 

The more I think about it. I probably don’t need to worry about high rpm driving. The longest stretch of highway I’ll probably run is 20 or so miles. I take it to town mostly and that’s only 5 miles of highway from my house.

 

Yeah, then 3.50 or deeper should be no issue and enjoy smokey burnouts!

 

Virile YOUT..LOVE IT🤣

 

I have a QP 9 inch in my 66. I use the 3.25's for cruising around but switch to 3.89's for planned track day stuff.
With a Hughes 2000 convertor it cruises 60 at about 2600 on a 255/60/15 on the 3.25's. With the Hughes 2500 it effectively turned my 3.89's into 4.30's, so think about that. The car is much happier with the 2000. Cam is 224/230 on a 400E.
I wouldn't over convertor a street car without an OD with lock up.

Consider the 3.25, 3.50 or 3.70 for the 9 inch and use a 1800-2200 convertor. You have a little more duration but it will be fine on a 425. My brothers W31 is factory auto with 3.91's and it is fine with the stock convertor, which is around 2300. That's 232 duration on a 350.

 

The poor man's quick change.

 

Yep, it only takes me about 30 mins with a lift. I have aluminum center sections so that helps.

 

Do you ever have trouble with the axle seals leaking when swapping that often?

 

Not yet