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Regear Rear End
#1
Hey everyone,

Just wanted to ask advice on re gearing the rear axle on my 71 mach 351C, 4 speed toploader. It came from the factory with a saddening 3.25 rear axle. I have around 450 HP coming from the engine and 1st gear is practically useless. What size gears would you recommend and will they fit in the 3.25 axle carrier my car came with? I've taken apart so many axles on my Jeeps that I could get it switched in around 2 hours taking my time.

Thanks
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#2
What transmission do you have?



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#3
4 speed toploader
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#4
If you frequently drive on the highway, 3.50 gears would be a good compromise. If you're just wanting to burn off some rubber around town, go for 3.73 gears. They're OK for occasional highway use, but your engine will be turning around 3,000 RPM at 60 mph. If you have a good tire budget and never drive on the highway you could even go as low as 4.10s.

You can play around with the spreadsheet in this thread to see what effect different gears will have.
https://www.7173mustangs.com/thread-tire...8#pid56388



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#5
A rear tire size or height would be helpful. Big difference in gears I'd recommend between a 26" tire and a 28".

IF you're pushing 450hp and everything behind the engine is factory stock, there are a couple upgrades you should make

1) Driveshaft to accommodate larger 1330 size u-joints

2) Nodular carrier (at a minimum) with 1330 yoke and 31 spline limited slip.

3) 31 spline axles from Moser/Strange etc


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#6
What cam are you running? RPM range? Seems to me that 450HP with 3.25 and a 4 speed you should be able to melt the tires as is unless your not coming into the powerband until 3000 RPM or so. What kind of torque numbers? Remember torque is what seat of the pants driving is all about.

'73 Grandé H Code. Headman long tube headers, T-5 Transmission, 3.70 Traclok, Lowered 1" all around, Aussie 2v heads w/ 2.19 intake, 1.71 exhaust, screw in studs, full roller cam 608/612 lift 280/281 duration LSA 112, Quick Fuel 750 CFM double pumper, AirGap intake.

'73 F code convertible. Bright red. Needs total restore. (IE HOT MESS)

- Jason
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#7
(01-28-2019, 07:35 PM)icejawa Wrote: Hey everyone,

Just wanted to ask advice on re gearing the rear axle on my 71 mach 351C, 4 speed toploader. It came from the factory with a saddening 3.25 rear axle. I have around 450 HP coming from the engine and 1st gear is practically useless. What size gears would you recommend and will they fit in the 3.25 axle carrier my car came with? I've taken apart so many axles on my Jeeps that I could get it switched in around 2 hours taking my time.

Thanks

it would help if you told us what your goal is. if 1st gear is useless now, then installing numerically higher gears will make 1st even more useless, so if you want 1st to be more useful, you want numerically lower gears, not higher ones.

.
[+] 1 user Likes barnett468's post
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#8
I was confused too. Hence my questions.

'73 Grandé H Code. Headman long tube headers, T-5 Transmission, 3.70 Traclok, Lowered 1" all around, Aussie 2v heads w/ 2.19 intake, 1.71 exhaust, screw in studs, full roller cam 608/612 lift 280/281 duration LSA 112, Quick Fuel 750 CFM double pumper, AirGap intake.

'73 F code convertible. Bright red. Needs total restore. (IE HOT MESS)

- Jason
  Reply
#9
Also. Where did you get the 450 HP number. Dyno? Did they provide an HP and torque curve? can you post?

'73 Grandé H Code. Headman long tube headers, T-5 Transmission, 3.70 Traclok, Lowered 1" all around, Aussie 2v heads w/ 2.19 intake, 1.71 exhaust, screw in studs, full roller cam 608/612 lift 280/281 duration LSA 112, Quick Fuel 750 CFM double pumper, AirGap intake.

'73 F code convertible. Bright red. Needs total restore. (IE HOT MESS)

- Jason
  Reply
#10
I'm running 3.89 gears-it is too much even with a tall tire for all but the most idiotic of drivers such as myself.

3.50 or 3.70 would be decent compromises. (" gears are also available in 3.60 and 3.64 ranges which would also be a nice compromise.

9" carriers fit all gears ratios-unlike some other designs that have different carriers depending upon the gear range.

I would encourage you to upgrade ujoints and yokes while you are at it, and if budget allows, axles. It is all easier when it is apart.

[Image: 1_01_07_15_8_53_18.png]

"I love my Hookers!" and "Get some Strange" probably have a different connotation to non automotive enthusiasts!
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