3.55 or 3.70 - Why?
Thread Starter
Melting Slicks
Joined: Jun 2009
Posts: 2,343
Likes: 4
From: Anderson SC
3.55 or 3.70 - Why?
I'm planning to change my rear end from a 3.08 to a 3.55 or 3.70.
I'm torn as to which one. I'm looking for opinions, and I'm hoping someone makes a convicing argument to tip the scales.
I plan to get overdrive later on - so I'll be able to make up the lost highway mileage etc.
-W
I'm torn as to which one. I'm looking for opinions, and I'm hoping someone makes a convicing argument to tip the scales.
I plan to get overdrive later on - so I'll be able to make up the lost highway mileage etc.
-W
Race Director
Joined: May 2006
Posts: 18,737
Likes: 2,582
I've had both. Depends on how you drive your car. The 3.70 will give you faster acceleration but cut down your top end and lower gas milesage. So your priorities will determine what you want. You say you will be getting overdrive so go with the 3.70.
Racer
Joined: May 2006
Posts: 259
Likes: 8
From: Shortsville NY
I'm planning to change my rear end from a 3.08 to a 3.55 or 3.70.
I'm torn as to which one. I'm looking for opinions, and I'm hoping someone makes a convicing argument to tip the scales.
I plan to get overdrive later on - so I'll be able to make up the lost highway mileage etc.
-W
I'm torn as to which one. I'm looking for opinions, and I'm hoping someone makes a convicing argument to tip the scales.
I plan to get overdrive later on - so I'll be able to make up the lost highway mileage etc.
-W
I have to agree with Melwiff, I don't think there will be much difference between the 3.55 and the 3.70. Depending on the number of gears in your tranny (and what each gear ratio is), highway RPM and gas milage are going to be the main difference I think.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Assuming manual trans here. (???) Your power curve and how and where you drive mostly would help determine the best answer, but it's hard to go very far wrong with either. If you've plenty of torque, by all means go with the 3.55:1. If torque isn't your engine's strong suit, the perhaps you'd be happier with the 3.70:1. Below are your RPM's at 65 MPH (assuming 27" diameter tires) for a few FDR's with a transmission geared at 1:1...
3.08:1 = 2487
3.36:1 = 2713
3.55:1 = 2867
3.70:1 = 2988
As you can see we're only talking about 121 RPM difference between the latter two. Do keep in mind that typical 1st gears in modern OD manual transmissions are deeper than those of older 4-speeds, so you need to take care that you don't end up too deeply geared when all is said and done. Also, you may be surprised that you very well could end up with better mileage around town with the deeper (than current) FDR, however counterintuitive it may seem (BTDT). Hope that helps.
TSW
3.08:1 = 2487
3.36:1 = 2713
3.55:1 = 2867
3.70:1 = 2988
As you can see we're only talking about 121 RPM difference between the latter two. Do keep in mind that typical 1st gears in modern OD manual transmissions are deeper than those of older 4-speeds, so you need to take care that you don't end up too deeply geared when all is said and done. Also, you may be surprised that you very well could end up with better mileage around town with the deeper (than current) FDR, however counterintuitive it may seem (BTDT). Hope that helps.
TSW
Drifting
Joined: Sep 2004
Posts: 1,625
Likes: 4
From: Miami FL
depends on how much overdrive you plan to get...
a 200R4 or 700R4 automatic usually have around 0.7 overdrive so I would suggest a 3.55
if you with with a 5 or 6 speed with say a 0.64 overdrive I would get a 3.70 or 3.55
all depends if you really plan to cruise any length of time at say 70mph
a 200R4 or 700R4 automatic usually have around 0.7 overdrive so I would suggest a 3.55
if you with with a 5 or 6 speed with say a 0.64 overdrive I would get a 3.70 or 3.55
all depends if you really plan to cruise any length of time at say 70mph
Tech Contributor
Joined: Aug 1999
Posts: 15,175
Likes: 3,989
From: Connecticut, USA
Just about every diff I built for OD used 373 gears. maybe 1 used 355 and a couple used 411's. There is going to be little RPM difference between the 355 & 370 or 373. The 373 is a better gear then the 370 strength wise. No difference with the speedo gears between 370 & 373's.
Drifting
Joined: Jul 2009
Posts: 1,534
Likes: 14
From: Dallas-Fort Worth TX
Clams, Im going with a 3.55 in a 700r trans. As mentioned probably not to much diff in this and 3:70/73 gears. With the 700r you got a great 1st gear and then nice OD in 4th.
I do not have the link, if I find I will post it, but BOWTIE overdrives has a site where you can type in the gear ratio and it will list out the rpms/speed for 350, 400 and 700 trans auto. If you got a muncie close ratio 4 speed like I used to have in my camaro back in the day. Then 3:73's will have you engine screaming at 80 mph in 4th.
I do not have the link, if I find I will post it, but BOWTIE overdrives has a site where you can type in the gear ratio and it will list out the rpms/speed for 350, 400 and 700 trans auto. If you got a muncie close ratio 4 speed like I used to have in my camaro back in the day. Then 3:73's will have you engine screaming at 80 mph in 4th.
Thread Starter
Melting Slicks
Joined: Jun 2009
Posts: 2,343
Likes: 4
From: Anderson SC
Assuming manual trans here. (???) Your power curve and how and where you drive mostly would help determine the best answer, but it's hard to go very far wrong with either. If you've plenty of torque, by all means go with the 3.55:1. If torque isn't your engine's strong suit, the perhaps you'd be happier with the 3.70:1. Below are your RPM's at 65 MPH (assuming 27" diameter tires) for a few FDR's with a transmission geared at 1:1...
3.08:1 = 2487
3.36:1 = 2713
3.55:1 = 2867
3.70:1 = 2988
As you can see we're only talking about 121 RPM difference between the latter two. Do keep in mind that typical 1st gears in modern OD manual transmissions are deeper than those of older 4-speeds, so you need to take care that you don't end up too deeply geared
3.08:1 = 2487
3.36:1 = 2713
3.55:1 = 2867
3.70:1 = 2988
As you can see we're only talking about 121 RPM difference between the latter two. Do keep in mind that typical 1st gears in modern OD manual transmissions are deeper than those of older 4-speeds, so you need to take care that you don't end up too deeply geared
-W
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Instructor
Joined: Sep 2006
Posts: 175
Likes: 11
From: Omaha ne
Assuming manual trans here. (???) Your power curve and how and where you drive mostly would help determine the best answer, but it's hard to go very far wrong with either. If you've plenty of torque, by all means go with the 3.55:1. If torque isn't your engine's strong suit, the perhaps you'd be happier with the 3.70:1. Below are your RPM's at 65 MPH (assuming 27" diameter tires) for a few FDR's with a transmission geared at 1:1...
3.08:1 = 2487
3.36:1 = 2713
3.55:1 = 2867
3.70:1 = 2988
As you can see we're only talking about 121 RPM difference between the latter two. Do keep in mind that typical 1st gears in modern OD manual transmissions are deeper than those of older 4-speeds, so you need to take care that you don't end up too deeply geared when all is said and done. Also, you may be surprised that you very well could end up with better mileage around town with the deeper (than current) FDR, however counterintuitive it may seem (BTDT). Hope that helps.
TSW
3.08:1 = 2487
3.36:1 = 2713
3.55:1 = 2867
3.70:1 = 2988
As you can see we're only talking about 121 RPM difference between the latter two. Do keep in mind that typical 1st gears in modern OD manual transmissions are deeper than those of older 4-speeds, so you need to take care that you don't end up too deeply geared when all is said and done. Also, you may be surprised that you very well could end up with better mileage around town with the deeper (than current) FDR, however counterintuitive it may seem (BTDT). Hope that helps.
TSW
I'll agree with all posted above except the last comment about the close ratio 4 spd screaming at 80. Correct me if I am wrong, but the close ratio 4 spd 4th gear ration is still 1:1 same as the others.
Myself, I usually steer a customer towards the 3:55 if it's a 3spd auto or 4spd. Both have a 1:1 final drive ratio. The 3:70's or lower gear typically wind a bit tight at 70 MPH especially on an extended cruise.
I have found that most of my customers have become accustomed to OD's and some find the higher RPM high way speeds annoying
Tim
Last edited by GDaina; Jul 31, 2009 at 02:15 AM.
Drifting
Joined: Jul 2009
Posts: 1,534
Likes: 14
From: Dallas-Fort Worth TX
Yep, I dont know, the 4 speed 4th gear probably was that ratio. But take that formula above and turn that 65 mph into 80 or 85 mph which is was most people cruise down the interstate at, packaged with headers, dual exhaust and 40 series flowmasters with dumps for 3.5 hrs down Interstate 20 , the rpm range is high with 3:73's. 
Burning Brakes
Joined: Dec 2003
Posts: 1,004
Likes: 7
From: Adelaide South Australia
It's interesting to hear a lot of comments about gas mileage and cruising rpm..... but not one post regarding terminal speed over the quarter mile.
oh well.... sign of the times, isn't it.
In regards to diff ratio if you intend to do any 1/4 mile action....
what you want is a diff ratio that allows you to use the full range of your power band.
If your motor runs out of real steam at 5000 rpm, then you might find a 3.73 diff a bit too short unless you use tall tyres.
But if you have a healthy motor spinning to 6000 real strong, then lower diff gears will improve your times slightly...
Get onto the speed / gear calculators such as
http://www.classicchevy5speed.com/RPM-Calculator.aspx
and work out what rpm you will cross the finish line at in 4th (1:1 ratio)
Not a big difference between 3.55 and 3.73, but there will be around 250 rpm difference in rpm crossing the line with typical 245/60R15 tyres
oh well.... sign of the times, isn't it.
In regards to diff ratio if you intend to do any 1/4 mile action....
what you want is a diff ratio that allows you to use the full range of your power band.
If your motor runs out of real steam at 5000 rpm, then you might find a 3.73 diff a bit too short unless you use tall tyres.
But if you have a healthy motor spinning to 6000 real strong, then lower diff gears will improve your times slightly...
Get onto the speed / gear calculators such as
http://www.classicchevy5speed.com/RPM-Calculator.aspx
and work out what rpm you will cross the finish line at in 4th (1:1 ratio)
Not a big difference between 3.55 and 3.73, but there will be around 250 rpm difference in rpm crossing the line with typical 245/60R15 tyres
Last edited by OzzyTom; Jul 30, 2009 at 07:32 PM.
Melting Slicks
Joined: Jul 2009
Posts: 2,091
Likes: 40
I'm planning to change my rear end from a 3.08 to a 3.55 or 3.70.
I'm torn as to which one. I'm looking for opinions, and I'm hoping someone makes a convicing argument to tip the scales.
I plan to get overdrive later on - so I'll be able to make up the lost highway mileage etc.
-W
I'm torn as to which one. I'm looking for opinions, and I'm hoping someone makes a convicing argument to tip the scales.
I plan to get overdrive later on - so I'll be able to make up the lost highway mileage etc.
-W
Because you can reuse the same carrier , and cahnge back easy if you grow to dislike stump pullers. The 3:36 is a good highway alxe. Some perforamce with out the heavy RPM that can get a bit annoying after a few hours. I had a 3:55 in Auto and didn't like it. But , 3:55 would be good for a four speed . Also the 3 :70 is good for for 4 speed cars too
Racer
Joined: Jul 2006
Posts: 277
Likes: 0
From: Big Spring TX
I would get the OD before I do the rear gears.
I put 3.73 gears in my nova with a th350 and while it was fun the first 30 minutes, being on the highway and turning 3200rpm's @ 70mph sure got old quick.
I put 3.73 gears in my nova with a th350 and while it was fun the first 30 minutes, being on the highway and turning 3200rpm's @ 70mph sure got old quick.
Le Mans Master
Joined: Apr 2001
Posts: 5,000
Likes: 1
From: Minneapolis, Mn USA
I did 3.08s with a 700R4 with a 2K stall.
It is the equiv of the TH350 with 3.73s in first gear and in OD on the highway at 70MPH, I'm at 2200 RPM. Works for me.
I've been around a car with TH400/4.56s and its fun, but gets old out on the road.
It is the equiv of the TH350 with 3.73s in first gear and in OD on the highway at 70MPH, I'm at 2200 RPM. Works for me.
I've been around a car with TH400/4.56s and its fun, but gets old out on the road.
Le Mans Master
Joined: Dec 2003
Posts: 5,727
Likes: 38
From: Fremont CA
Good thing you don't want a 3.90 gear set, Richmond gear is scrapping all their 3.90 sets based on the problems we had installing one for my Vette (sent it back to Richmond for threaad fit analysis and bolt pattern tolerance check, seems to have failed the exam
.) I got the last good one in stock all others are going into the scrap bin! I will say that Richmond does back their product but, my friend Dave Herlinger at Corvette Repair (corvetterepair.org) is out a bunch of hours of labor trying to get the gearset to fit and is bemoaning the fact that the pattern came in right away at the Richmond specified pinion depth which was too good to be true. Hopefully this replacement gear set installs quickly and effortlessly. Next on the agenda is the track only 4.33 gearset with 30 spline side axels (yokes) that Gary=GTR1999 is going to help me get going so I can DUMP my clutch with slicks on the Vette.
.) I got the last good one in stock all others are going into the scrap bin! I will say that Richmond does back their product but, my friend Dave Herlinger at Corvette Repair (corvetterepair.org) is out a bunch of hours of labor trying to get the gearset to fit and is bemoaning the fact that the pattern came in right away at the Richmond specified pinion depth which was too good to be true. Hopefully this replacement gear set installs quickly and effortlessly. Next on the agenda is the track only 4.33 gearset with 30 spline side axels (yokes) that Gary=GTR1999 is going to help me get going so I can DUMP my clutch with slicks on the Vette.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
What he said and I'll add
I'll agree with all posted above except the last comment about the close ratio 4 spd screaming at 80. Correct me if I am wrong, but the close ratio 4 spd 4th gear ration is still 1:1 same as the others.
Myself, I usually steer a customer towards the 3:55 if it's a 3spd auto or 4spd. Both have a 1:1 final drive ratio. The 3:70's or lower gear typically wind a bit tight at 70 MPH especially on an extended cruise.
I have found that most of my customers have become accustomed to OD's and some find the higher RPM high way speeds annoying
Tim
http://tandmautomotive-omaha.com/
I'll agree with all posted above except the last comment about the close ratio 4 spd screaming at 80. Correct me if I am wrong, but the close ratio 4 spd 4th gear ration is still 1:1 same as the others.
Myself, I usually steer a customer towards the 3:55 if it's a 3spd auto or 4spd. Both have a 1:1 final drive ratio. The 3:70's or lower gear typically wind a bit tight at 70 MPH especially on an extended cruise.
I have found that most of my customers have become accustomed to OD's and some find the higher RPM high way speeds annoying
Tim
http://tandmautomotive-omaha.com/
Last edited by TheSkunkWorks; Jul 31, 2009 at 12:37 AM. Reason: typo
Safety Car
Joined: Nov 2008
Posts: 3,720
Likes: 554
From: Seattle Washington
Sounds like in the long Run your going to build a Stroker 427. A Storker Motor will make good Torque and therefore will pull a Low gear rato on the Freeway well. As your planning on ending up with a 5-6 Speed with over drive plan on useing it for a low 1st gear for acceleration off the line. Your probably going to be traction limeted anyway.If you have a small motor with a lot of cam then your going to want a High gear ratio cause in overdrive the motor isn't going to make enough Torque to pull it down the freeway. So what I'm saying is that You might just want to leave the rear alone and install the Trans of choise first.
Pro
Joined: Mar 2009
Posts: 703
Likes: 2