65fastbackman

I have a 65 fastback mustang and am putting a 408w stroker motor into it which will have approximately 400 hp and about the same torque. The car originally had an 8" open rear end in it with I think 3.00 gears. I just bought a 9" housing/axles and my question is what gear ratio should I use for when I buy a new 9" third member/center section? I have a 245/45/17 wheel in the rear, T-5 trans with I believe a 2.95 first gear? Anyway, I think the rear end ratio for the 8" was approximately a 3.00 but I want to have bigger gears for better acceleration but still be able to drive it on the freeway at 60+ mph and not be at high rpms. I'm looking at a posi unit that has 3.50 gears... any feedback or input?

Bigorangek9

IMO, I think the 3.50 will be just about right, that will give you good pickup plus not too bad on interstate. That's just me, I'm sure you'll get alot of different answers. I tried 3.73's in a firebird once & I hated interstate driving with a 4spd. An extra gear would have helped. I settled for 3.42 in the end & it was perfect.

AdderMk2

i dont know the ratios for a 9" but, I run 3.73's in my 94 (8.8) and its very well mannered for highway driving. I think you have a 3.77? option. That is what I would recommend.

GT350R Klone

Good advice above. Somewhere between 3.5-3.8 depending on how you want to drive it
I had 3.89 with a similar set-up and it was too much gear.
I had 3.50 and it was an excellent all around, street driven gear.

Starfury

3.73's would be great for street and freeway driving.

Your problem is going to be that T5. A 408 is going to have more than enough torque to snap the tailshaft in that transmission. Even the aftermarket T5z models are only rated to 330ft-lbs. If you're planning on using sticky street tires, I'd start looking for a stronger transmission, or maybe an AstroPerformance rebuild for the T5.

65fastbackman

While I don't disagree with you, I also don't know exactly who to believe on this topic. I've heard some people use a T-5 and they break it while power shifting behind a stock 5.0 liter motor. And then I've heard that some guys put a stock T-5 behind a 400 plus hp and equal amount of torque, 11-12 second race car and do pass after pass and have no issues. So I'm not exactly sure who to believe, any insight or words of wisdom?

GT350R Klone

65, yea, that is the good and bad thing about the internet. Ton of information, opinions and experiences....many different. I've beat plenty of T5's in mustangs and my cobra replica and have not hurt any of them. I do know that those in the cobra circles do avoid the T5's with big torque motors. If you plan on beating it hard, I would go with the tremec.

zmetalmilitia

It really comes down to the entire powertrain system. RPM's all depend on the gearing of the gears in your tanny working with the gears in the rear end. If your concerned about high RPM's on the highway, but want fast off the line acceleration, your setup is good but you need that coveted 5th gear, like 0.82 or 0.64. There are RPM calculators out thee too that will tell you what to expect your rpm's will be at certain speeds.

I plan on going with the tremec tko600 with a 5th gear ratio of 0.64 or 0.82.

Here's a cool website that may help you.

http://www.keislerauto.com/wizard.html

65fastbackman

Thank you to everyone for your feedback/input on the rear end and transmission. I'm wanting to build this car properly but I'm only 24 years old and don't exactly have $2k to go out and buy a brand new Tremec transmission, especially after buying a $3k stroker motor and just buying a rear end set up off ebay. I ended up getting a pretty good deal on ebay though, got the housing and rear axles at the proper width for the 65 fastback from ebay for $160 plus $50 for shipping and then I got a rebuilt/reman 9" third member with 3.50 gears and a posi for $350 plus $75 for shipping which I thought was a great deal considering new third members with similar gearing go for over $1500 on mustangs plus. Anyway, I'm wondering what everyone else's opinion on the T-5 is and what they think. Like I said, I'd love to go with a Tremec but I can't really afford it at the moment. Also, the T-5 has a 10 spline input shaft and a 28 spline output shaft, does Tremec offer their transmissions with those splines? Thanks again everyone

Waffles

I have a 408W with a 3.50 posi. Love it. Just need to finish my AOD now cause highway speeds have me running at like 5000 rpm.