3.55's or 3.80's (Rear Gear)?

[Jayru 17]

I decided to save cash and ditch the 9" dreams for now... So I'm taking the plunge this week and buying a rebuilt/new 8" third member with a new ford track lock and my choice of gears.

 

The guy building it says 8" technology has come a long way the past few years (gears/track lock/etc) and it should reliably handle 500+ HP (way less than i have at the moment).

 

I've always had 3.73's in my fox body stangs and found them to be best for a motor with the HP/RPM range to about 6K with a 5 speed. Really keeps it in the sweet spot.

 

They don't make 3.73's for an 8"... So my instinct's saying 3.80's.

 

But the Adult in me is saying sacrifice a little performance and go 3.55's for better hwy cruising/MPG's. But the kid in me really want's those 3.80's! I do plan to drive the car a lot with a few long trips here and there (and my goal of autocrossing at some point).

 

The RPM difference is apparently only 200 rpm higher with the 3.80's over 3.55's. But that's a lot at 70 on the highway no?

 

What's your opinion? What are you all running?

[Jayru 17]

I decided to save cash and ditch the 9" dreams for now... So I'm taking the plunge this week and buying a rebuilt/new 8" third member with a new ford track lock and my choice of gears.

 

The guy building it says 8" technology has come a long way the past few years (gears/track lock/etc) and it should reliably handle 500+ HP (way less than i have at the moment).

 

I've always had 3.73's in my fox body stangs and found them to be best for a motor with the HP/RPM range to about 6K with a 5 speed. Really keeps it in the sweet spot.

 

They don't make 3.73's for an 8"... So my instinct's saying 3.80's.

 

But the Adult in me is saying sacrifice a little performance and go 3.55's for better hwy cruising/MPG's. But the kid in me really want's those 3.80's! I do plan to drive the car a lot with a few long trips here and there (and my goal of autocrossing at some point).

 

The RPM difference is apparently only 200 rpm higher with the 3.80's over 3.55's. But that's a lot at 70 on the highway no?

 

What's your opinion? What are you all running?

[StephenC 10]

I have 3.55 in my 8" but not only does the gear ration matter what type of trans is in it? I have a C4 in mine and even with 3.55 it still screams down the highway and jumps off the line.

[StephenC 10]

I have 3.55 in my 8" but not only does the gear ration matter what type of trans is in it? I have a C4 in mine and even with 3.55 it still screams down the highway and jumps off the line.

[Fordrevhead 29]

yeah I have 3.73's in my Fox body with a built full manual C4, shift kit and 2800 stall convertor... and I about topping out shy in the 1/4 mile! lol I ended up going with a taller tire so I could drive on the highway. My first cruise prior to that I was running 4600 rpm at 72mph. Not too great on gas that way.

[Fordrevhead 29]

yeah I have 3.73's in my Fox body with a built full manual C4, shift kit and 2800 stall convertor... and I about topping out shy in the 1/4 mile! lol I ended up going with a taller tire so I could drive on the highway. My first cruise prior to that I was running 4600 rpm at 72mph. Not too great on gas that way.

[Jayru 17]

yeah I have 3.73's in my Fox body with a built full manual C4, shift kit and 2800 stall convertor... and I about topping out shy in the 1/4 mile! lol I ended up going with a taller tire so I could drive on the highway. My first cruise prior to that I was running 4600 rpm at 72mph. Not too great on gas that way.

 

Haha! Wow, 4600! That's nuts.

 

I have a Z rated T5 5 speed, so i have .7 overdrive. With 3.55's i should be at around 2400-500 @ 70mph. 3.80's would have me around 2700.

 

I am currently getting around 17mpg on the hwy with a set of 3.00's @1900rpm.

[Jayru 17]

yeah I have 3.73's in my Fox body with a built full manual C4, shift kit and 2800 stall convertor... and I about topping out shy in the 1/4 mile! lol I ended up going with a taller tire so I could drive on the highway. My first cruise prior to that I was running 4600 rpm at 72mph. Not too great on gas that way.

 

Haha! Wow, 4600! That's nuts.

 

I have a Z rated T5 5 speed, so i have .7 overdrive. With 3.55's i should be at around 2400-500 @ 70mph. 3.80's would have me around 2700.

 

I am currently getting around 17mpg on the hwy with a set of 3.00's @1900rpm.

[Fordrevhead 29]

Sounds like you know the math. Without doing the math, off the top of my head, I think I would go with the 3.55's for the usage you state for the following reasons:

 

1. figure out your ratio in 1st, I think the 3.80's might render 1st practically useless in a drag race situation and on the street just an annoying extra shift. I'd end up raising the rpms and launching in 2nd all the time which is just clutch wear around town. No biggy for autocross, but...

 

2. fuel economy. from my experience depending on cam, induction, exhaust,etc there's a couple of magic numbers where fuel economy drops.. I dont believe it is a straight line or steady curve graphing rpms to fuel economy. 2000-2100 is one of the marks and the second is 2400-2500.

 

3. I do a lot of cruising around the backroads (55mph). I think at this speed 4th gear would be a lot more in the power band with the 3.55's. Anyway, you see what I am getting at, do the math for the speeds you drive. If you have an exhaust drone that you like see what speed you'll be at to get that rpm and in what gear. Or vice-versa if you have a drone you dont like see if you can avoid. Of course if you ever plan a tire size change you have to factor that in as well.

[Fordrevhead 29]

Sounds like you know the math. Without doing the math, off the top of my head, I think I would go with the 3.55's for the usage you state for the following reasons:

 

1. figure out your ratio in 1st, I think the 3.80's might render 1st practically useless in a drag race situation and on the street just an annoying extra shift. I'd end up raising the rpms and launching in 2nd all the time which is just clutch wear around town. No biggy for autocross, but...

 

2. fuel economy. from my experience depending on cam, induction, exhaust,etc there's a couple of magic numbers where fuel economy drops.. I dont believe it is a straight line or steady curve graphing rpms to fuel economy. 2000-2100 is one of the marks and the second is 2400-2500.

 

3. I do a lot of cruising around the backroads (55mph). I think at this speed 4th gear would be a lot more in the power band with the 3.55's. Anyway, you see what I am getting at, do the math for the speeds you drive. If you have an exhaust drone that you like see what speed you'll be at to get that rpm and in what gear. Or vice-versa if you have a drone you dont like see if you can avoid. Of course if you ever plan a tire size change you have to factor that in as well.

[Jayru 17]

The numbers i was quoting were from the rear end guy (and it seems he was wrong). After some digging tonight... I found a gear ratio calculator online and the specs for my "Z" rated T5.

 

I have a 2.95 1'st gear and a .63 5th gear overdrive

 

My tires are 245x60x15 which = 26.6 diameter

 

It seems with 3.55's I'd be around 1970rpm

 

With 3.80's I'd be at around 2100rpm

 

With a low 2.95 1st gear - 3.80's may be better for holeshot acceleration.

 

55mph cruising in 4th would have me at 2600rpm with the 3.80's (right where my cam starts to make power).

 

Thanks for making me dig a little deeper revhead! I was leaning 3.55's but it looks like the 3.80's would be perfect I'm thinking.

[Jayru 17]

The numbers i was quoting were from the rear end guy (and it seems he was wrong). After some digging tonight... I found a gear ratio calculator online and the specs for my "Z" rated T5.

 

I have a 2.95 1'st gear and a .63 5th gear overdrive

 

My tires are 245x60x15 which = 26.6 diameter

 

It seems with 3.55's I'd be around 1970rpm

 

With 3.80's I'd be at around 2100rpm

 

With a low 2.95 1st gear - 3.80's may be better for holeshot acceleration.

 

55mph cruising in 4th would have me at 2600rpm with the 3.80's (right where my cam starts to make power).

 

Thanks for making me dig a little deeper revhead! I was leaning 3.55's but it looks like the 3.80's would be perfect I'm thinking.

[Jayru 17]

Hey Revhead, i chickened out at the last minute and went with 3.55's!

 

I cruised down the highway doing 80 the whole way to pick up the rear today. The tach never went above 1900... The whole time i was thinking, man i can't give this up! i should still be able to do 70 under 2K with the 3.55's. The kid in me is disappointed though! Haha!

[Jayru 17]

Hey Revhead, i chickened out at the last minute and went with 3.55's!

 

I cruised down the highway doing 80 the whole way to pick up the rear today. The tach never went above 1900... The whole time i was thinking, man i can't give this up! i should still be able to do 70 under 2K with the 3.55's. The kid in me is disappointed though! Haha!

[MikeStang 247]

How much you spending on the Built 8", I mean really you should be able to build a 9" for same price huh?

I bought a Center Section which contained a 28 spline Trac Loc and N case for 400 bucks, then spent 150 for complete rebuild kit and did it myself...had to buy a tool or two which I needed and New Gears, im into it now for 750.00 and its all New and ready to go....Stock axles that came with the housing I bought, which was 100 bucks and I sold the center section out of it and...So total into the rear is 800.00 to do it ALL....You have that much in your 8" ??

Mike

[MikeStang 247]

How much you spending on the Built 8", I mean really you should be able to build a 9" for same price huh?

I bought a Center Section which contained a 28 spline Trac Loc and N case for 400 bucks, then spent 150 for complete rebuild kit and did it myself...had to buy a tool or two which I needed and New Gears, im into it now for 750.00 and its all New and ready to go....Stock axles that came with the housing I bought, which was 100 bucks and I sold the center section out of it and...So total into the rear is 800.00 to do it ALL....You have that much in your 8" ??

Mike

[70mstang 11]

Something I heard the other day was that a 9" will drag you down if you aren't pushing over 350 HP, is that true?

[70mstang 11]

Something I heard the other day was that a 9" will drag you down if you aren't pushing over 350 HP, is that true?

[Jayru 17]

Something I heard the other day was that a 9" will drag you down if you aren't pushing over 350 HP, is that true?

 

It's more rotating mass/heavier, so it seems like it would be true. Not sure how much drag/loss we're really talking about though.

 

Although just going to an aluminum drive shaft in a fox body Mustang makes a difference you can feel. So you never know.

[Jayru 17]

Something I heard the other day was that a 9" will drag you down if you aren't pushing over 350 HP, is that true?

 

It's more rotating mass/heavier, so it seems like it would be true. Not sure how much drag/loss we're really talking about though.

 

Although just going to an aluminum drive shaft in a fox body Mustang makes a difference you can feel. So you never know.

[Fordrevhead 29]

Something I heard the other day was that a 9" will drag you down if you aren't pushing over 350 HP, is that true?

 

Well, it's heavier, lol. Other than that I have never heard such a thing.

 

I suppose I could do a torque wrench calibrated torque test to compare friction out in my shop one day.

 

I have killed or been in the car or truck that killed rear diffs about 5 times. I built the 8.8 Traction Lok in my Fox and with a mild 302 and heavily modified C4 and stall converter I have completely destroyed the clutch disks in that in about 20k miles of street use. I currently have .085" end play so that needs to come apart again! Needless to say, to me the extra weight is worth it for a bullet proof rear diff.

[Fordrevhead 29]

Something I heard the other day was that a 9" will drag you down if you aren't pushing over 350 HP, is that true?

 

Well, it's heavier, lol. Other than that I have never heard such a thing.

 

I suppose I could do a torque wrench calibrated torque test to compare friction out in my shop one day.

 

I have killed or been in the car or truck that killed rear diffs about 5 times. I built the 8.8 Traction Lok in my Fox and with a mild 302 and heavily modified C4 and stall converter I have completely destroyed the clutch disks in that in about 20k miles of street use. I currently have .085" end play so that needs to come apart again! Needless to say, to me the extra weight is worth it for a bullet proof rear diff.

[70mstang 11]

Thanks, I'll keep on with the 9", and who knows I may beef up the engine more as time goes on. We're never really done with our cars, are we?

[70mstang 11]

Thanks, I'll keep on with the 9", and who knows I may beef up the engine more as time goes on. We're never really done with our cars, are we?

[Pakrat 1,043]

I don't think anyone can debate that a 9" is way stronger than an 8", the real argument is where the threshold is and if you really need a 9" or not. I think a lot of folks tend to overbuild there cars and upgrade way more stuff than they will ever need, sure it's all cool but if it never gets put to the test it's really just money wasted.

 

I can't remember how long ago it was, could be 1-3 years ago but there was a great 67' fastback build in Mustang Monthly where the goal wasn't so much to build a monster but do a modest 351 build and cut weight as apposed to just increasing cubes and horses to see how it stacked up against a bloated big block and the main reason I remember this article so well is because they went into some detail about why they chose a built 8" over a 9", it really changed my mind on thinking I needed a 9". A lot of the legend around the 9" being bullet proof and the 8" being insufficent has to do with irresponsible and unbalanced builds over the years and not so much the quality of the parts. I tried to find that article on line but came up empty, I'll bet I can find it easier just thumbing thru pile of mags this weekend.

 

I did find this article though which may have a lot of the same info in it. http://mustangmonthly.automotive.com/44018/mump-0009-ford-8-inch-rearend-build/index.html

 

Granted if the dollar amounts were equal then it would be foolish not to go with a 9" but the 8" doesn't have to be a lost cause.