Suggestions for rear diff gears - 1969 Mach 351W 4v

[cfauvel]

My Mach 1 was born with a 3spd TopLoader and 3.00:1 open diff (not going to lie ...that combo sucked)

A while back I swapped in a 1993 or so Tremec T5 (I love it BTW) , but the performance is still pretty low....so this year I am going to get a whole complete center section for my 9" rear diff and I'm vacillating between the the 3.50:1 , 3.70:1, and 3.75:1 Eaton TrueTrac helical-gear carrier (the TrueTrac is what I am going to go with)

What are you guy's thoughts?

the T5 should have these gear ratios
1 - 2.95
2 - 1.94
3 - 1.34
4 - 1
5 - .63 OD
UPDATE 04/09/2024) correction...my T5 is 1990 variant
1 - 3.35
2- 1.99
3 - 1.33
4 - 1
5 - .68

That taller 1st gear with a 3.70 rear ratio, might be much....3.70X 3.35 = 12.395:1 effective ratio


I've been playing with this calculator to see what some of my numbers would be.

Spicer Transmission Ratio RPM Calculator

Use the Spicer Transmission Ratio RPM Calculator to determine your engine’s RPM based on the transmission gear ratio, tire height, MPH, and ring and pinion gear ratio.

spicerparts.com

I have an old Stewart Warner tach from the 60s on the steering column that I am suspect of in regards to accuracy.

Also note: I am running too small of tires 225/60r14......which should be around 24.6" diameter
I'll probably go to a more appropriate tire size of 225/70r14 ....which should be around 26.4" diameter

I'm looking for off the line quickness than top speed or 1/4 mile speed....more like better 0-60 times....(right now it is super embarrassing, but I still get thumbs up, because of the car, not its performance)

Thanks

 

[Noobz347]

What are your guy's thoughts?

--and--

 

[cfauvel]

LOL, ok not going to go the SuperCharger route...

 

[Noobz347]

LOL, ok not going to go the SuperCharger route...

But, look at the money you'd save in gears?

A 3:27 or 3:73 would go nicely with the T5. If you didn't like the 3:00 at all, I would lean toward the 3:73s.

 

[RPM Mach One]

A 3.35 first gear won't get you across the intersection. Your 3.00 rear gear would go nicely with that 3.35 first gear.

 

[manicmechanic007]

Get the 3.70 rear gears

 

[cfauvel]

Get the 3.70 rear gears

Yeah I think I've resigned to the 3.70 ratio....I have to pull the car out and undo the rear U-joint to re-measure the amount of play I have at the slip yoke.

This will steer me to the right pinion yoke....I have a long 1310 currently, but I think my amount of slip is pretty small...MAYBE 1/2" or less. A short 1310 is 1" shorter so that would put me in a 1" + (unknown) worth of slip which is probably a bit too much depending how long the slip yoke is and how far the splines go into the slip yoke...

I haven't had any troubles with the current slip yoke, driveshaft, and pinion yoke....but if the slip is way less than 1/2" then I'm afraid of bottoming out the slip yoke onto the output shaft of the transmission...

That is the hold up right now. Which pinion yoke to go with.

 

[AeroCoupe]

I know my car is a 93 but driveline is driveline. I have a 92 WC T5 with the same gear ratios as yours has and if I could redo my rear gear selection I would have gone with 3.55’s. Rear tires are a 275/40/17 drag radials which are 25.67” and first is just a little too much with the 3.73’s. Car has right at 370 rwhp for reference. 3.55’s would have been my preference but I don’t drive the car enough to justify the gear change cost.

Great looking Sportsroof man!

 

[cfauvel]

know my car is a 93 but driveline is driveline. I have a 92 WC T5 with the same gear ratios as yours has and if I could redo my rear gear selection I would have gone with 3.55’s. Rear tires are a 275/40/17 drag radials which are 25.67” and first is just a little too much with the 3.73’s. Car has right at 370 rwhp for reference. 3.55’s would have been my preference

Ugh....decisions decisions

note: the 1969 Mach1 351W probably has less than 200 RWHP...probably closer to 150-175 Hp

 

[AeroCoupe]

Yeah but it probably has as much torque as my 331.

 

[cfauvel]

Indeed....indeed