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Discussion Starter · #1 ·
Although I’d love to have a 9in spending the money on new axles and housing and diff and 3 link, at my power levels it’s not worth dropping 10 grand when at my power level is below 300 I’ll stick with my 8in for a while
So I figured I’d order some gears and a traction device
I currently have a t5z with the 2.95 first gear and 2.80 open
While I love the mpgs it’s just so sluggish and can’t use 5th until like 80 mph
So currently looking at a 3.80 with a truetrack from quick performance
Does anyone recommend I do anything different?
 

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I think 3:80 would be about a perfect setup. I have a t-5 with a .68 OD and 3:55 rear end. And OD cruising around 70 leaves a lot to be desired.
 

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Discussion Starter · #3 ·
I think 3:80 would be about a perfect setup. I have a t-5 with a .68 OD and 3:55 rear end. And OD cruising around 70 leaves a lot to be desired.
It seems like the non z spec guys like the 3.55s but the z spec has a little bit “better” first so I think bumping it up to 3.80 and I would be cruzing at 62mph @2k rpm and 93mph @3k rpm so I can be in the 2-3k rpm range for the majority of highway speed limits
 

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I have 3.55 rear gears and a "non-Z" transmission with a Z gearset, except for fifth. This results in a 0.59 fifth. It works, but fifth is really too tall for my liking.

3.80 would be a good choice. It's still the equivalent of only 2.35:1 rear gears with a four speed.

Another option is a 0.80 fifth gear set in your trans but that of course only affects fifth and won't give you the added benefit of a reduced first gear overall ratio. A "short" fifth is nice though when driving on an entertaining road with some hills and curves where you want to shift between fourth and fifth without a big drop in RPM. With a normal T5, the difference between gears is 1-2: 35%, 2-3: 31%, 3-4: 34%; 4-5: 37%
 

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I run a T-5z with 3.55 and short tires (225/60x14). I find it good all around. I can shift into 5th at 45 and it will run at 1,350 rpm. If I were putting new gears in, I would probably bump up to 3.80 and go to a taller tire.
 

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I have 3.89's with a t5 from an 86 5.0 and 26" tall tires. RPM's on the freeway are pretty high. I could give you a ride if you want to see what its like before I pull the drivetrain.
 

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T5z, 3.80 posi with 26” tires in my 66. I don’t use 5th until about 55-60 on the highway, about 1600-1800 rpm, but at that point I’m just moseying along enjoying the drive.
 

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67 Fastback T5 331 TCI Frt End, Canted 4 link rear susp
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T5z with 3.55 gears and 235/45/17 rear tires. Can easily shift into 5th @ 45-50 mph. Also, have read plenty of info that the 8" diff can handle upwards of 500 hp if built with quality parts. Of course, that depends on how you're driving the car.
 

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my '66 has a t5z with 3.80's. i run a 235/60/14, 5th gear at 60 is 2100. the 3.80 for me is about perfect.
 

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Discussion Starter · #10 ·
Alright sounds like the 3.80 is the choice, thanks for all the info, now what about traction devices trutrack or posi?
 

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In my '65, I run a T5z with 3.73 gears (8.8") and 245/45/17 rear tires. It works well. I'm building a 9" with a Yukon traction lock and will use 3.89 gears in that.
 

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I'm running a 3:80 with a true track and a T5z. I like it, good acceleration light to light and I can cruise at 75MPH and be right at 2500RPM.
 

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TrueTrac = yes.
3.80's? Not in my book.
I'd go with 3.25 or 3.40, tops. Frankly, I'd stay with 3.00's if that's what's in there. You spend a LOT more time shifting from 1st to 2nd and back to 1st in normal driving than 4th to 5th to 4th.... So WHAT if you don't use 5th except zipping down the 4 lane?
 

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3.80 with that 2.95 1st seems like a good combination. You might even be able to get away with 4.11 as well if you don't mind shifting a little sooner and if you don't do too much high speed cruising(though I would avoid 4.11 if you ever plan on turbocharging the car, for turbo I would choose 3.55 instead...well in realty for turbo I would choose a wide ratio Toploder T-4, not a 5 speed at all). In a high revving NA SBF application with a 2.95 1st I would choose 3.80 myself though. With the steeper 3.27 1st that would be another story completely
 
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I'm running 3.70 in a 9" with TruTrac diff, with a WC T5 (standard Fox ratios). 1st gear is somewhat short I guess, but not obnoxiously so. I'd like a taller 1st gear to help with traction like you'd get with your Z. Even with a summer 255/40/17 rear tire, I can't get it to hook at WOT in 1st. 2nd is still good for low 60 MPH range, and 70 is in the low 2000 RPM range in 5th. I am a mostly back road cruiser, maybe an auto-x this year, etc. The short gearing is awesome, especially with my engine combo (306, roller, E303, Edelbrock RPM heads, air gap, Sniper EFI, full exhaust). I also like the mechanical LSD...my first vs trac-lok clutch and it's been good so far (about 1,500 miles on it so far). If you go Z I would go 3.80 (or comparable range for your axle) and you'll love it. Good luck!
 

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The options would be Ford Traction Lock, Detroit Locker or Trutrac. (I know everybody calls it "Posi" and I have no problem with that. It's just that if you try to find a "Positraction" for a Ford diff, you'll be disappointed. :D

I have a Truetrac in my '70 Mach 1. It's great. Around town it's like a single track; no clunking, no chirping, no nothing. If I nail it in a straight line it's like a Detroit locker. Best of both worlds in my book.
 

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Discussion Starter · #20 ·
TrueTrac = yes.
3.80's? Not in my book.
I'd go with 3.25 or 3.40, tops. Frankly, I'd stay with 3.00's if that's what's in there. You spend a LOT more time shifting from 1st to 2nd and back to 1st in normal driving than 4th to 5th to 4th.... So WHAT if you don't use 5th except zipping down the 4 lane?
Currently I have a 2.80 and It’s way to steep for 1st and reverse takes a lot of clutch modulation and rpm’s not to stall and then I’m going 20mph in reverse
 
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