3.25 rear gears vs 3.55 ?
#1
Hey Guys,

I'm contemplating whether I should change my stock rear differential gears from 3.25 to 3.55. I don't do much highway driving with my 69 Mach and if I do, I wouldn't travel more than an hour on the highway. Would I feel a noticeable difference going to a 3.55 gear? I want more seat off your pants feel off the line.

351w with weiland intake
crane hmv-272-2-nc cam
edelbrock 600cfm
super comp headers
FMX trans
electronic ignition (pertronic ignitor III with flamethrower III)
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#2
I was not happy with my gears in my first Mustang and wanted the same off the line launch power as you do. I ended up changing to 3.89 gears and now much happier with the launch power and overall performance of the car. However, without an overdrive there are some long distance highway driving limitations. I ended up changing to a taller tire to compensate for some lower RPM's when driving 60 MPH for longer distances. However, the car still runs around 3200 RPMS at 60mph.

Changing from 3.25 to 3.55 you may not notice the launch power you are trying to achieve but overall 3.55 gears are a nice set-up for good performance and still providing good drivability.

I hope this helps you with your decision.
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#3
If it's a 9" (which it should be) I think the ratio is 3.50. Aside from that trivia. Here are my thoughts from my experience on a street driven car that will see little highway driving.

1) For an automatic trans car without an overdrive and a close to original size tire, the 3.50 ratio is a good choice. Much more and the engine RPM will be a little too high during highway driving (my opinion).

2) For a manual trans car without an overdrive and close to original size tires the 3.70 or 3.89 is a good choice. With a manual trans the slippage from a torque convertor does not exist which is why a lower gear ratio is possible.

I recently went from a 3.89 to a 3.70 to try to bring the RPM down a little and not sacrifice much acceleration. With 3.89's , 4sp manual trans, and the stock tire size, the engine RPM was just like Mustangmike mentioned. Just a little too high for my preference.

Back in high school my older brother's 69 Mach 1 had a 3.50 rear axle ratio. It was an automatic trans, 351W car. On that car going from a 3.00 to a 3.50 ratio made a huge improvement. It was surprisingly quick.

Regards,
Mike
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#4
Thanks for the info. I'm thinking 3.5 gears for me. I'm a little confused on what I have to buy. Do I just need the gear?

Cjponyparts has this kit which is more than I want to upgrade
http://www.cjponyparts.com/currie-3rd-me...73/p/CE-V/
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#5
I noticed the CJ Pony item your link goes to is an 8" third member and also a Traction-Loc. If your car is a Mach 1 it should have a 9 inch rear axle. The 9 inch rear axle was standard on 351W motor Mustangs and the 351W was the smallest motor available in a 1969 Mach 1.

So, I am assuming you have a standard 9" ford rear axle and it's not a Nodular Case third member nor have a Daytona Pinion Housing or be any type of Traction-Loc. Basically you need a Ring and Pinion Gear set of your choice, and an installation kit like the one below.

Installation kit: https://www.summitracing.com/parts/mgr-r9rmkt/overview/

If you have an existing Traction-Loc posi traction unit or are purchasing a new one it needs specific ring gear bolts. They have a smaller hex size, 5/8" instead of 3/4", to fit into a counter bore. There are also long and short bolts. I don't know what determines the length but I have always needed the longer bolts.

Bolts:
https://www.summitracing.com/parts/rat-1305/overview/

Typical Traction-Loc unit for 28 spline axles. A unit for 31 spline axles would only be needed if a previous owner upgraded to 31 spline axles. 28 Spline axles were standard on 351W based cars.

https://www.summitracing.com/parts/fms-m.../overview/

Installation kits come with a pinion bearing preload crush sleeve. If you desire, you can opt for a solid pinion spacer instead of a crush sleeve. They take more time to setup the pinion bearing preload. But, do a better at maintaining the bearing preload. https://www.summitracing.com/parts/rmg-0.../overview/

I would stay away from Richmond brand gears. Richmond also has a lower end line. I don't recall the brand but I would avoid those as well. Richmond gears tend to be noisy. I have had good luck with Motive's Performance series. I think Mustangmike had good luck with US Gear. Yukon gear is also supposed to be a good brand.

If you can follow instructions and have some basic mechanical skills you can setup a 9" rear axle. Some tools are needed that you might already have.
1) Dial indicator to measure backlash,

2) Inch pounds bar type torque wrench or dial type torque meter to check bearing preloads by measuring the torque needed to rotate the pinion gear at the pinion nut.

3) Pinion depth measuring tool or depth micrometer helps to set the pinion depth instead of the trial and error method.

4) 1/2 inch drive foot pounds torque wrench

Mike
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