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Old 01-16-2012, 09:41 PM   #1
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Question 3.31/3.55/3.73 Rear Axle Ratio?

Please excuse my ignorance in Rear Axle ratio's, but I am a bit confused on which one to choose for a 2012 Mustang GT. I am not knowledgeable on how the three different Rear Axle's would affect the performance of the car(acceleration, shifting times, etc.), so could someone explain the differences of each Rear Axle and how it would it affect the performance of the car?

I'm trying to choose solely on which ratio would benefit me most, performance wise, so which one would you recommend?

Thanks in advance, and once again, I apologize for my ignorance
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Old 01-16-2012, 09:47 PM   #2
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if performance is the only consideration then the 3.73s will be your best bet.

a non technical explanation as I understand it - shortening the final drive ratio essentially lessens your potential max MPH (that you'll never reach, anyways), in exchange for shorter gears. this gets you into the power band much faster and requires more shifting, 1st gear goes by in the blink of an eye. from a non technical stand point it feels like you're adding gobs of torque in the low end, it feels much snappier off the line.

you will cruise at higher rpms and as such your MPG will go down 1-2.

I heavily suggest you go drive a 3.31 and a 3.73 in person to see the difference for yourself. I promise you there are many local versions of each mustang near you. 3.55s would feel in between the two and typically need to be custom ordered.
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Old 01-16-2012, 10:04 PM   #3
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Also, which transmission are you optioning in?
Are you staying with a manual? (MT82)
Or are you optioning in the automatic? (6R80)

The reason why this is significant is because the gear ratios in each gear box are significantly different.
The automatic (6R80) has a much deeper 1st gear, so off the line, with 3.73:1 rear gears, it is like the manual (MT82) with a 4.10:1 rear gear (4.10:1 is not even available as a factory option).
So deep is the automatic's 1st gear that it comes factory stock with 3.15:1 rear gears with 3.31:1 and 3.55:1 as options (I do not know if the 3.73:1 is an option for the automatic).

In direct comparison: a GT with automatic with 3.31:1 rear gear will feel like it pulls significantly harder than the manual with 3.31:1.

So, whenever you go test driving, dont just try manuals with 3.31:1, & 3.73:1 gears, also test drive automatics with 3.31:1 and 3.55:1 gears and see which car pulls harder.

Aside from the rear gears, rumor has it that the automatic 6R80 is designed to take a flywheel torque load of 800ft-lbf. I do not know if this is true because I have not seen any Ford publication that states this.
And actually, Ford does not even publish the MT82's 380ft-lbf flywheel rating either. And nor does Getrag, the sub-contractor that makes the MT82 for Ford. I only know this because Ford engineers who designed the Ford Coyote 5.0 engine said this in an interview with a car magazine. And it makes sense that the Coyote 5.0 with 390ft-lbf flywheel torque rating does not grossly exceed the MT82 380ft-lbf rating. Heck, even the Roadrunner (Boss 302 engine) had its peak torque reduced to 380ft-lbf to match the MT82's rating.
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Old 01-16-2012, 10:59 PM   #4
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3:73 hands down. stick a set of fat meats on the rear to minimize tire spin and you will fly...your always in your power band with 3:73's.
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Old 01-17-2012, 08:30 AM   #5
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Quote:
Originally Posted by Entaille View Post
if performance is the only consideration then the 3.73s will be your best bet.

a non technical explanation as I understand it - shortening the final drive ratio essentially lessens your potential max MPH (that you'll never reach, anyways), in exchange for shorter gears. this gets you into the power band much faster and requires more shifting, 1st gear goes by in the blink of an eye. from a non technical stand point it feels like you're adding gobs of torque in the low end, it feels much snappier off the line.

you will cruise at higher rpms and as such your MPG will go down 1-2.

I heavily suggest you go drive a 3.31 and a 3.73 in person to see the difference for yourself. I promise you there are many local versions of each mustang near you. 3.55s would feel in between the two and typically need to be custom ordered.
Thanks for the reply. When you say "shorter gears", are you reffering to the 3.31's, or the 3.73's?

Thanks
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Old 01-17-2012, 09:24 AM   #6
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Quote:
Originally Posted by JIM5.0 View Post
Also, which transmission are you optioning in?
Are you staying with a manual? (MT82)
Or are you optioning in the automatic? (6R80)

The reason why this is significant is because the gear ratios in each gear box are significantly different.
The automatic (6R80) has a much deeper 1st gear, so off the line, with 3.73:1 rear gears, it is like the manual (MT82) with a 4.10:1 rear gear (4.10:1 is not even available as a factory option).
So deep is the automatic's 1st gear that it comes factory stock with 3.15:1 rear gears with 3.31:1 and 3.55:1 as options (I do not know if the 3.73:1 is an option for the automatic).

In direct comparison: a GT with automatic with 3.31:1 rear gear will feel like it pulls significantly harder than the manual with 3.31:1.

So, whenever you go test driving, dont just try manuals with 3.31:1, & 3.73:1 gears, also test drive automatics with 3.31:1 and 3.55:1 gears and see which car pulls harder.

Aside from the rear gears, rumor has it that the automatic 6R80 is designed to take a flywheel torque load of 800ft-lbf. I do not know if this is true because I have not seen any Ford publication that states this.
And actually, Ford does not even publish the MT82's 380ft-lbf flywheel rating either. And nor does Getrag, the sub-contractor that makes the MT82 for Ford. I only know this because Ford engineers who designed the Ford Coyote 5.0 engine said this in an interview with a car magazine. And it makes sense that the Coyote 5.0 with 390ft-lbf flywheel torque rating does not grossly exceed the MT82 380ft-lbf rating. Heck, even the Roadrunner (Boss 302 engine) had its peak torque reduced to 380ft-lbf to match the MT82's rating.
Thanks for the assistance Jim. Personally, I am choosing the MT28 manual over the automatic. I feel that I would enjoy the the manual much more than the automatic, but thats just my personal preference.

I wish Ford would have choose a manual transmission that wasent inferior to that of the automatic, but I would guess it was to save cost and keep the price lower than the competition(Camaro and Challanger)

The thing that raised concern, was the problems owners were having with there MT28, with the rough shifting, gear lockouts, etc. The NHTSA concluded there investigation of the manual problems, and stated that the problems only affected a small percentage of owners. I was happy to hear the small percentage of problems, but I was also dissapointed because of how Ford and the NTHSA handled those who had problems. There was no real fix.

Back to the gearing ratio's, would there be a difference in acceleration between the 3.31 gears and 3.73 gears? Is the advertised 0-60mph within 4.5 seconds obtainable with the 3.31 gears?
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Old 01-17-2012, 09:40 AM   #7
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Quote:
Originally Posted by dominant1 View Post
3:73 hands down. stick a set of fat meats on the rear to minimize tire spin and you will fly...your always in your power band with 3:73's.
Thanks for the reply. Is there a major difference between the acceleration of a 3.31 geared Mustang, compared to a 3.73 geared Mustang?
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Old 01-17-2012, 04:44 PM   #8
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Yes there is a major difference. at 390 ft lbs in 5th gear, the 3.31 transmit 1290 ft lbs to the rear axles, where the 3.73 transmit 1454 ft lbs at peak torque. It is the equivalent of a 12.7% increase in hp/torque across the entire rpm range (50 ft lbs and 52 hp at peak numbers). Downside is shifting at lower speeds, mpg, and of course top speed.
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Old 01-17-2012, 06:28 PM   #9
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3.73 ALL day! Top speed isn't even a concern. In a 1/4 mile, a stock 5.0 MT82 will tap about 112mph and that is topping out 4th gear. There are still 2 gears left. I've had mine up to about 130 on the hwy in 5th and still pulling. How often do you REALLY see anything over that, even on a race track? I ran 12.959 at 110.9mph on my FOURTH run in this car on a crappy track and minimal traction. Car had 700 miles on it. ALL STOCK! 3.73's are the way to go. Hands down. And, I could be wrong, but I THINK the auto is only available w/ 3.15's or 3.31's.
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Old 01-17-2012, 07:19 PM   #10
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The smaller the number, i.e. 3.31, the taller the gear...the larger the number, the shorter the gear.

Hope that's clear as mud.
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Old 01-17-2012, 07:19 PM
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