Regear Rear End

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icejawa

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1971 Mach 1 M Code,2003 Mach Azure Blue
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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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-sizes--5980?pid=56388#pid56388

 
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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

 
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.

 
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
Click to expand...
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.

.

 
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.

 
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.
Click to expand...
I have 3:89's in mine also. They were in the car when I got it. I don't have my motor back yet but it will be behind a TKO 600 with a .64 5th gear (Motor should be around the 450-500HP range). Most of my local driving I'll see a high of 45-50MPH so I'll be spending most of my time in 4th but if I need to head out on the highway I'll be good to go. I'm also concerned about how useful or useless my 1st gear will be...

 
Following this thread with interest. A useless 1st may mean different things to people. Useless can mean that the gear is so short that you end up using 2nd most of the time to move the car from a stop. However, useless can also mean that it is hard to get the tires to spin. Therefore, is useful somewhere in between? I bet we get many different answers.

Edit PS: I have a TKO600 and should be close to 500hp. I drove it a little with the 3.00 gears open differential. I was able to easily spin the tires just by pressing the gas pedal. I decided to move up to a 3.50 gear, tracloc and 31 splines in a Strange center section. We will see how it drives.

 
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Following this thread with interest. A useless 1st may mean different things to people. Useless can mean that the gear is so short that you end up using 2nd most of the time to move the car from a stop. However, useless can also mean that it is hard to get the tires to spin. Therefore, is useful somewhere in between? I bet we get many different answers.

Edit PS: I have a TKO600 and should be close to 500hp. I drove it a little with the 3.00 gears open differential. I was able to easily spin the tires just by pressing the gas pedal. I decided to move up to a 3.50 gear, tracloc and 31 splines in a Strange center section. We will see how it drives.

Hey Tony, my setup when done will be similar to yours. What ratio is your 5th gear in your TKO?
Click to expand...
 
Following this thread with interest. A useless 1st may mean different things to people. Useless can mean that the gear is so short that you end up using 2nd most of the time to move the car from a stop. However, useless can also mean that it is hard to get the tires to spin. Therefore, is useful somewhere in between? I bet we get many different answers.

Edit PS: I have a TKO600 and should be close to 500hp. I drove it a little with the 3.00 gears open differential. I was able to easily spin the tires just by pressing the gas pedal. I decided to move up to a 3.50 gear, tracloc and 31 splines in a Strange center section. We will see how it drives.

Hey Tony, my setup when done will be similar to yours. What ratio is your 5th gear in your TKO?
Click to expand...
Click to expand...
5th is 0.67

 
Following this thread with interest. A useless 1st may mean different things to people. Useless can mean that the gear is so short that you end up using 2nd most of the time to move the car from a stop. However, useless can also mean that it is hard to get the tires to spin. Therefore, is useful somewhere in between? I bet we get many different answers.

Edit PS: I have a TKO600 and should be close to 500hp. I drove it a little with the 3.00 gears open differential. I was able to easily spin the tires just by pressing the gas pedal. I decided to move up to a 3.50 gear, tracloc and 31 splines in a Strange center section. We will see how it drives.
Click to expand...
With a 3.25 rear-end I assumed that with 450 horsepower there is limited torque below 3,000 RPM, which means riding the clutch to start moving from a dead stop, unless you rev it to around 4,000 and release the clutch, which can be fun, but not appreciated when you're at a crosswalk at a busy intersection.

With a wide-ratio toploader 4,000 RPM in first would equate to around 35mph.

 
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The 1st gear is useless as in I start from a stop in 2nd all the time, didn't know bigger gears would be the same. I just get worried when cruising down the street 30 mph and the engine sounds like its at 4000 rpm. I'm keeping the normal size tires and they do bust loose easy in any gear but I will upgrade axle shafts and u joints since I've already done it like 10 times between my 2 Jeeps. I had it wheel dynoed with 450 HP and 430 ft/lb torque, I'll have to get the sheet uploaded to here. I've actually never had it on the freeway always cruise easy, but I do dump the clutch sometimes around ricer kids in their jap cars.

 
Are you 100% sure it still has the original 3.25 gears in it? Sounds like they may have been swapped out for taller gears already. I would not go numerically any lower than 3.25. Mine had the original 2.75 gears in it and it was a dog.

 
No idea guess I'll look up how to determine which gears are in there when I open the case up

 

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