Rear End Gear Advice

[Doc76]

Running 3.08’s in the back on a stocker 302. Not by choice
It’s very low idling and slow out of the hole. In fact I haven’t even got it out of 3rd.
I’ll be yanking the 302 in the near future for a 347 stroker.
Was looking for some feedback on what you guys would swap to as for the rear gearing.

My thought is 373 or 355

I occasionally do highway cruising but mostly in town, stop and go and I like to “get on it” frequently during our cruises.

Any advice would be appreciated.

[RoadRacer]

I went from 3.27 to 3.73 but it’s a big discussion that factors in how you want your car to drive. How high your motor revs, tire height etc etc

Here’s a quote I kept: “you want 9.5-11 range for snappy acceleration - 2.87(first gear ratio) * 3.73(rear ratio)= 10.7”

Also play around with the tremec calculator and look at speeds in each gear. (Even if not using a tremec) https://www.tremec.com/calculadora.php

[egchewy79]

3.55 for me paired with a T5Z trans.

[JMD]

The combination of a light weight vehicle and an engine with lots of low end torque, go with a lower (numerical) gear ratio than you normally would. I've got a mildish 302 with about 340tq at the crank, 3.55 rear gear, and TKX. Personally I wouldn't go with a larger rear gear than what I have as I'm already fighting wheel spin at low speed. I can literally let my foot off the clutch at idle and the car moves right along...and you're going to have a lot more low end torque with a 347. I can even move out from a stop in second gear without any trouble. That being said, if your use is going to be different (track, drag racing with slicks, etc) or of you have a larger first gear that changes things...but for a fun street machine, I would not go with anything more aggressive than a 3.55. If I had a 347 I would probably prefer a 3.31 to the 3.55.

The tremec gear ratio calculator is great. Plug some numbers in and see what you like. Just keep in mind these cars behave differently than your normal muscle car that's carrying around another thousand pounds. These cars have less traction due to lower weight so don't need to rev as high as heavier cars to get moving.

Also, you are planning to change variables on both ends of the drivetrain, not just one. A 347 might feel nice with that 3.08, or a 3.55 or 3.73 might feel great with your current engine. I would discourage making drastic moves on both ends at the same time.

[BEAR-AvHistory]

Happy with a 3:55 the best IMHO middle ground rear. Select a trans with a taller OD ratio for good highway driving. Example TKO 600 0.64:1 street and a 0.82:1 performance.

[CraigS]

Doc we need to know the ratios in your trans. Until then a couple of thoughts. More rear gear as in 373 means you run out of first gear quickly. I don't mean on a max accel run, I mean just driving around, stop, then go to turn left across a 3 lane road, and you need to shift half way across. OTOH as JMD says "I can literally let my foot off the clutch at idle and the car moves right along...and you're going to have a lot more low end torque with a 347." That makes for a nice easy driver in traffic and you basically don't bother going full throttle in 1st since it just lights up the tires. You use 2nd and 3rd as your fun gears. Over the years in my MkII I went from a 351 w/ a 308 gear to a 331 gear and then a 408. A year after the 408 I did a retrofit 2015 IRS w/ a 315 gear. The move 308 to 331 was perfect for the 351 but not quite so perfect w/ the 408. Then the IRS 315 I was thinking I would probably be changing to a 331 but a few years later I realized it was just fine as is. A thing to note playing w/ gear calculators. Look at your speed in each gear at 6000 w/ say the 308, then look at a one step change to a 327 or 331. You will find a surprisingly small difference. Or look at your rpm at 70 w/ both gears. Again a small difference. You say 'near future' for the 347. I would get that in the car first and then think about rear gear.

[Rebostar]

My 2018 Mustang GT with a Roush Stage II Supercharger makes 750 HP and 740 TQ now. It came from Ford with 465 HP and 450 TQ with IRS & 355 gears. Perfect gearing for both applications. I also put 350 gears in my 427 FE 1970 Cougar backed by a C6. The 350 gears keep it quick off the line and reasonable (about 1800 rpm) at 60 mph. I also put the Motive 355's in my 9" on my Roaster. Strongly recommend 350 or 355.
Good luck and Happy Trails
Allyn

[Doc76]

The combination of a light weight vehicle and an engine with lots of low end torque, go with a lower (numerical) gear ratio than you normally would. I've got a mildish 302 with about 340tq at the crank, 3.55 rear gear, and TKX. Personally I wouldn't go with a larger rear gear than what I have as I'm already fighting wheel spin at low speed. I can literally let my foot off the clutch at idle and the car moves right along...and you're going to have a lot more low end torque with a 347. I can even move out from a stop in second gear without any trouble. That being said, if your use is going to be different (track, drag racing with slicks, etc) or of you have a larger first gear that changes things...but for a fun street machine, I would not go with anything more aggressive than a 3.55. If I had a 347 I would probably prefer a 3.31 to the 3.55.

The tremec gear ratio calculator is great. Plug some numbers in and see what you like. Just keep in mind these cars behave differently than your normal muscle car that's carrying around another thousand pounds. These cars have less traction due to lower weight so don't need to rev as high as heavier cars to get moving.

Also, you are planning to change variables on both ends of the drivetrain, not just one. A 347 might feel nice with that 3.08, or a 3.55 or 3.73 might feel great with your current engine. I would discourage making drastic moves on both ends at the same time.

Ok thanks for this input.
Honestly I’ll probably not moved to the 347 for some time however am planning to pull the rear end to for a disc brake conversion and to properly paint the rear end next month. I figured because I’d have it out swapping the gears will be easy for some pep until the 347 comes to life.
With 3.08 it cruises really nice. Super low rpm but honestly it’s a dog out of the hole and I don’t even get a chance to move into 4th. In fact come to think of it I have never driven in 5th. That’s how low the RPMs are even on the freeway.

[CraigS]

In that case go 355. Over the years that has proven to be the most often used.

[weendoggy]

Doc we need to know the ratios in your trans. Until then a couple of thoughts. More rear gear as in 373 means you run out of first gear quickly. I don't mean on a max accel run, I mean just driving around, stop, then go to turn left across a 3 lane road, and you need to shift half way across. OTOH as JMD says "I can literally let my foot off the clutch at idle and the car moves right along...and you're going to have a lot more low end torque with a 347." That makes for a nice easy driver in traffic and you basically don't bother going full throttle in 1st since it just lights up the tires. You use 2nd and 3rd as your fun gears. Over the years in my MkII I went from a 351 w/ a 308 gear to a 331 gear and then a 408. A year after the 408 I did a retrofit 2015 IRS w/ a 315 gear. The move 308 to 331 was perfect for the 351 but not quite so perfect w/ the 408. Then the IRS 315 I was thinking I would probably be changing to a 331 but a few years later I realized it was just fine as is. A thing to note playing w/ gear calculators. Look at your speed in each gear at 6000 w/ say the 308, then look at a one step change to a 327 or 331. You will find a surprisingly small difference. Or look at your rpm at 70 w/ both gears. Again a small difference. You say 'near future' for the 347. I would get that in the car first and then think about rear gear.

Boy, does this bring back memories of the bickering on gearing 25yrs ago. 3:55 or 3:73 and those found out just what you mentioned, useless 1st gear, or the whine of the engine at highway speed. I went with 3:27 and it works just fine. The car is light, 2300#, and can pull itself easily. While others were smoking their tires, I was just launching. I even have that gearset in my Coyote powered '02 Mustang with TKX and it is a blast, on or off track. Same goes for the Cobra powered 347 and TKO500. It's just a personal preference.

[Mark Reynolds]

Are your sure you have a 3.08 currently? Your description sounds more like a 2.73 Rear End.
I have 3.27 in my 340hp Ford Racing 302 and I would not change it.

[svassh]

Interesting calculator I've been using for another car I'm doing a T5 swap on. Think about your speed when you typically do your first shift, maybe 30mph, maybe you like to shift sooner. Do you want to be at 4500RPM at 30mph in first etc.

https://purperformance.com/p-29669-rpm-calculator.html

[TBull]

I agree with what's mostly been said hear, but also know what power range you'll be in with your new engine. I'm running over 580 at the rears now and I really enjoy the the 3.27 gear ratio and it calmed the car down from the 3:55. I have plenty of torque to launch out of the hole but it is predictable. Personally I wouldn't go over 3:55. My last one was at 450 RWHP with 3:55's and just a blip of the throttle and I was shifting. I also planned on tracking the car so I got the TKO600RR transmission as well with the .82 fifth gear. A lot of thought can go into the decision, but all well worth it in the end I feel. Best of luck and let us know what you decide.

[Doc76]

Are your sure you have a 3.08 currently? Your description sounds more like a 2.73 Rear End.
I have 3.27 in my 340hp Ford Racing 302 and I would not change it.

I’m going to check again next time it’s in the air but ya, it’s a dog

Yes 3.08

[Doc76]

I agree with what's mostly been said hear, but also know what power range you'll be in with your new engine. I'm running over 580 at the rears now and I really enjoy the the 3.27 gear ratio and it calmed the car down from the 3:55. I have plenty of torque to launch out of the hole but it is predictable. Personally I wouldn't go over 3:55. My last one was at 450 RWHP with 3:55's and just a blip of the throttle and I was shifting. I also planned on tracking the car so I got the TKO600RR transmission as well with the .82 fifth gear. A lot of thought can go into the decision, but all well worth it in the end I feel. Best of luck and let us know what you decide.

Pretty sure I’ll go 3.55. Power range on the new engine will be ~550hp at crank but built to boost if I ever choose to.
I am removing the rear end after a test fit of my rear disc conversion kit to blast/paint and may upgrade to 31 spline at the same time. Not sure what I’ll find when I open up the rear. In doing so I’m not sure on carrier.
What are thoughts on brand?
Eaton, Torsen, Auburn, or?
Looking for something that is rebuildable and doesn’t compromise the driving experience.

[StangRacer]

Pretty sure I’ll go 3.55. Power range on the new engine will be ~550hp at crank but built to boost if I ever choose to.
I am removing the rear end after a test fit of my rear disc conversion kit to blast/paint and may upgrade to 31 spline at the same time. Not sure what I’ll find when I open up the rear. In doing so I’m not sure on carrier.
What are thoughts on brand?
Eaton, Torsen, Auburn, or?
Looking for something that is rebuildable and doesn’t compromise the driving experience.

I am waiting for my rear housing to come back from being narrowed and just went through making these choices...

I had an old 28 spline traction loc I was planning to use and began rebuilding it last week. Unfortunately, I found the spider gears had a good amount of wear. I found replacement spider gears to be ~$110. I called a buddy of mine that drag races and uses a spool hoping he may have his old traction loc laying around. I told him my situation and it turned out he did in fact have his old 28 spline traction loc... he then told me he had a brand new 31 spline traction loc still in the box he purchased a few years ago before he decided to go to a spool. He let me have the new 31 spline traction loc for the cost of new spider gears.

My original intention was to always use the traction loc as my car is just a driver. I will only have ~300 horsepower at the crank and I don't ever plan to take it for a track day or autocross...

When I found the worn spider gears I did some research on the commonly available carriers. It appears the Eatons are rebuildable as are the Yukon which appear to be a copy of the Eaton. I never considered the Auburns as I recall several folks from my early Mustang days having issues with them. The Torsen carriers are very nice, but I am not sure how they would hold up to your horsepower level if you ever did a drag race style launch. There are no clutches to wear in a Torsen. Another carrier to check out is the Detroit TrueTrac. The TrueTrac is a "worm gear" carrier like a Torsen. On a side note, two different "Camaro Mustang Challenge" national champions ran welded up traction locs. I would have never believed it, but I have done work on both of their cars and have seen it with my own eyes. They even run that set up in the rain!! Their talent as a drivers is on a different level...

As far as the gear ratio, I went with the 3.55 at the suggestion of many here on the board.

[Doc76]

I am waiting for my rear housing to come back from being narrowed and just went through making these choices...

I had an old 28 spline traction loc I was planning to use and began rebuilding it last week. Unfortunately, I found the spider gears had a good amount of wear. I found replacement spider gears to be ~$110. I called a buddy of mine that drag races and uses a spool hoping he may have his old traction loc laying around. I told him my situation and it turned out he did in fact have his old 28 spline traction loc... he then told me he had a brand new 31 spline traction loc still in the box he purchased a few years ago before he decided to go to a spool. He let me have the new 31 spline traction loc for the cost of new spider gears.

My original intention was to always use the traction loc as my car is just a driver. I will only have ~300 horsepower at the crank and I don't ever plan to take it for a track day or autocross...

When I found the worn spider gears I did some research on the commonly available carriers. It appears the Eatons are rebuildable as are the Yukon which appear to be a copy of the Eaton. I never considered the Auburns as I recall several folks from my early Mustang days having issues with them. The Torsen carriers are very nice, but I am not sure how they would hold up to your horsepower level if you ever did a drag race style launch. There are no clutches to wear in a Torsen. Another carrier to check out is the Detroit TrueTrac. The TrueTrac is a "worm gear" carrier like a Torsen. On a side note, two different "Camaro Mustang Challenge" national champions ran welded up traction locs. I would have never believed it, but I have done work on both of their cars and have seen it with my own eyes. They even run that set up in the rain!! Their talent as a drivers is on a different level...

As far as the gear ratio, I went with the 3.55 at the suggestion of many here on the board.

Why did you end up needing to narrow your rear end?
Did you add c-clip eliminators or?

[Doc76]

I am waiting for my rear housing to come back from being narrowed and just went through making these choices...

I had an old 28 spline traction loc I was planning to use and began rebuilding it last week. Unfortunately, I found the spider gears had a good amount of wear. I found replacement spider gears to be ~$110. I called a buddy of mine that drag races and uses a spool hoping he may have his old traction loc laying around. I told him my situation and it turned out he did in fact have his old 28 spline traction loc... he then told me he had a brand new 31 spline traction loc still in the box he purchased a few years ago before he decided to go to a spool. He let me have the new 31 spline traction loc for the cost of new spider gears.

My original intention was to always use the traction loc as my car is just a driver. I will only have ~300 horsepower at the crank and I don't ever plan to take it for a track day or autocross...

When I found the worn spider gears I did some research on the commonly available carriers. It appears the Eatons are rebuildable as are the Yukon which appear to be a copy of the Eaton. I never considered the Auburns as I recall several folks from my early Mustang days having issues with them. The Torsen carriers are very nice, but I am not sure how they would hold up to your horsepower level if you ever did a drag race style launch. There are no clutches to wear in a Torsen. Another carrier to check out is the Detroit TrueTrac. The TrueTrac is a "worm gear" carrier like a Torsen. On a side note, two different "Camaro Mustang Challenge" national champions ran welded up traction locs. I would have never believed it, but I have done work on both of their cars and have seen it with my own eyes. They even run that set up in the rain!! Their talent as a drivers is on a different level...

As far as the gear ratio, I went with the 3.55 at the suggestion of many here on the board.

Any thoughts on a Ford Performance Trac-Loc 31 spline?
Looks like it’s about half the cost of an Eaton which would free up some cash for 31 spline axles, should I “need”/ want to

Also anyone recommend rear gear oil?
75w90, 75w140…..?

[StangRacer]

Doc,

I had to have the rear narrowed because I am going to use pin-drive wheels. I am just using the standard 8.8 ends. C-clip eliminators are not a good idea for a car that goes around turns as they tend to leak. Although, several years ago I vaguely recall reading something about someone who had developed c-clip eliminators for a Chevy 12-bolt that were suitable for road racing. It may have been Strange or Mark Williams... I never really gave it much interest since I had no intention of using a Chevy 12-bolt... If you are going to do serious road racing with an 8.8 the best set up would be to convert it to a full floater. There are several "kits" available from "roundy round" shops that could be adapted to an 8.8 that make it not too bad as far as cost is concerned. The second best set up would be to have 9 inch ends put on the 8.8. Many refer to this as a "semi-floater".

[StangRacer]

Any thoughts on a Ford Performance Trac-Loc 31 spline?
Looks like it’s about half the cost of an Eaton which would free up some cash for 31 spline axles, should I “need”/ want to

Also anyone recommend rear gear oil?
75w90, 75w140…..?

I like the Ford traction loc carriers. I used a 28-spline traction loc in my CMC race car and it worked well. I rebuilt it using the FRPP carbon clutches and did not use the spring in the middle of the carrier. It was suggested to leave the spring out as it would help the car with turn in... I have no idea how it compared to having the spring installed as I never raced with it installed.

I am using the 31-spline traction loc in my build. As I posted earlier, my car is just a driver so I am not concerned about having the best lap times. The traction locs have proven to be fairly durable, perform well, and are easy/inexpensive to rebuild when the time comes. 31 spline axles will easily handle 550 hp. I used 31 spline axles with a spool in my 3,000+ lb drag car with 600+ horsepower launching with drag radials. It shook the tires many times and the axles were always looked like new when I would go through things at the end of each season.

[Entropy]

Just wanted to chime in on the gear ratios thing (only a Mustang owner and a Cobra planner at this point).

I found this site really useful when mathing out possible combinations:

https://www.blocklayer.com/rpm-gear

Output includes specific shift points and a table of RPM vs speed and a graph of rowing through the gears.

And just to reinforce how important it is to look at the transmission ratios: first gear in my 2015 MT-82 is 3.66, first gear in one of the T56 options is 2.66.

I personally like that my 3.55 Mustang + MT-82 is "one gear every 30 mph" in gears 1-4 - makes it more engaging just putting around town, commuting, etc. Roushcharged so plenty of torque down low.