DIFF GEARING

[smidee 10]

I have a 70 hard top with a 351c t5 and a 9" I want to change my gearing  from 2.75 to something better usage will be a little of everything no strip work I thinking of either 3.25 or 3.55

it has a 3.35 1st gear and a mild 351clevo  looking for some feedback

[Mike65 475]

I am putting a T-5 in my 69 Coupe & I put 3.50 gears w/trac loc in mine, unfortunately it is still not derivable so I can not give you any feedback. I did have 3.50 gears in a 68 Coupe with mild 302 & C-4 & it had good get up & go.

[Vicfreg 771]

I have a '68 coupe with a T-5 and 3..73 gears and I love the way it drives.  Goes like hell in gears 1-4, and is tolerable on the highway.

For my 1970 Convertible build, with a C-6 Automatic, I went with a 3:25 posi, as I want the car to be cruiser.  

I would say that the 3:55 (or 3:50) gears are a great choice, and you will get the acceleration you want, plus some decent RPMs on the highway.   

If you want to give yourself a headache, you can read up on the whole debate on the "hunting" versus "non-hunting" gear ratios.   I have attached a excerpt from one of my reference books on this.  

I would run whatever rear you like and just have a loud exhaust....

 

 

[barnett468 418]

if you have an 8'' diff i would use 3.43 for your app it will be a massive change from what you had. 373 nearly render 1st gear useless for daily driving.

[smidee 10]

I have a 9"open center

 

[barnett468 418]

8 minutes ago, smidee said:

I have a 9"open center

 

i would use 3.25 or 3.40 in your case.

 

3.25 if you cruise it most of the time and drive long distances at high speed and 3.40 if you want it to be extremely sporty but still do some high speed driving.

 

https://www.strangeengineering.net/product-category/make-and-model/ford/9in-center-sections-components-1/standard-gear-sets-1/

[RPM 1,190]

Wouldn't you want to know the T5 o.d. ratio before getting your rear gears? Isn't  the T5 od ratio .92 or .81? You gotta multiply the od ratio by a rear ratio that will give you the final drive ratio you want. That's how I'd do it anyway. 

 

[smidee 10]

OD IS .675

 

[RPM 1,190]

Ya, that's the same OD gear ratio as the AOD. The way I see it, if you're going to the trouble of installing an OD trans you should take advantage of its gearing. With 3.73 rear gears the OD turns it into 2.50's. I ran 2.50's on my Florida trip last October which I really liked. But 2,50 is the biggest gear I'd be happy with. If 3.73's make the T5's first gear useless 90% of the time, just start out in 2nd gear.

[MN69Grande 203]

1 hour ago, RPM said:

Ya, that's the same OD gear ratio as the AOD. The way I see it, if you're going to the trouble of installing an OD trans you should take advantage of its gearing. With 3.73 rear gears the OD turns it into 2.50's. I ran 2.50's on my Florida trip last October which I really liked. But 2,50 is the biggest gear I'd be happy with. If 3.73's make the T5's first gear useless 90% of the time, just start out in 2nd gear.

I've got a C4 with a Gearvendors OD.  Final Drive Ratio of C4 + Gearvendors = .78 with 3.55 gears I've got a 2.77 total effective ratio and I find it very drive-able at all speeds.  So I'd have to agree that 3.73s would be a fun option to drive something a little more all-around might be 3.55s.  Personally since these are mostly weekend cars I like a little more pep so I wouldn't go lower the 3.40s.  I'm at 2500 rpm at 70 mph so the motor is going but it's not ridiculous. 

With the 5 speed and 3.73s you would be at around 2300 rpm at 70.  With 3.55s your down to 2000 rpm.   What are you getting now with the 2.75s about 1690 rpm?

If you do go with the higher gears though I would suggest going Truetrac if the cost doesn't kill you.

[skidmarky 50]

I'm running 3.25's with my T5 in my 69 Mach1, 351w. I turn 2100 rpm at 75mph, so you could probably step up to the 3.55's for a bit better acceleration.

[barnett468 418]

2 hours ago, RPM said:

Ya, that's the same OD gear ratio as the AOD. The way I see it, if you're going to the trouble of installing an OD trans you should take advantage of its gearing. With 3.73 rear gears the OD turns it into 2.50's. I ran 2.50's on my Florida trip last October which I really liked. But 2,50 is the biggest gear I'd be happy with. If 3.73's make the T5's first gear useless 90% of the time, just start out in 2nd gear.

3.73 does make 1st nearly useless 90% of the time, especially in bigger cube engines like a 351, therefore, if you start out in 2nd to alleviate the problem, well you now just paid a boatload of money for a 5 speed trans you only use 4 of the forward gears on.

[Vicfreg 771]

I finally went to a T-5 Z (Ford Motorsport M-7003-Z), after using 2 other "used" T-5 transmission from Fox Body Mustang GTs..  It has a 2.95 first gear and a 0.63 Overdrive.  With my 3.73 rear, it is much more drivable than the previous T-5s I used.  I have never started in 2nd gear.     

Some good info here on the variety of gear ratio's that were offered with the T-5 over the years.  

 

http://www.allfordmustangs.com/Detailed/349.shtml

 

 

 

[Midlife 814]

22 hours ago, Vicfreg said:

I have a '68 coupe with a T-5 and 3..73 gears and I love the way it drives.  Goes like hell in gears 1-4, and is tolerable on the highway.

For my 1970 Convertible build, with a C-6 Automatic, I went with a 3:25 posi, as I want the car to be cruiser.  

I would say that the 3:55 (or 3:50) gears are a great choice, and you will get the acceleration you want, plus some decent RPMs on the highway.   

If you want to give yourself a headache, you can read up on the whole debate on the "hunting" versus "non-hunting" gear ratios.   I have attached a excerpt from one of my reference books on this.  

I would run whatever rear you like and just have a loud exhaust....

 

 

Most interesting.  I have never run across this explanation for gear whine due to common ratios in any of my automotive documentation. 

[Vicfreg 771]

I had never seen it either.  It is in a book I found on rebuilding 9" Ford rear ends.  I decided on a 3:25 rear anyway, in spite of the "common factor".   I think it is probably a 2nd or 3rd order harmonics issue, which I left buried in some textbook long ago in engineering school.  

The one thing it did bring to mind was that for instance on a 3:00 rear, how with a 13 tooth pinion and a 39 tooth ring gear, that the pinion gear teeth always contact the same ring gear teeth every third revolution.  Hmmmm......

[barnett468 418]

if you are using 3.50 or numerically lower gears and you can hear them whine, your gears are worn, or improperly set up, or they are crappy gears, or there is the wrong oil or no oil in your diff. those are the only options.

the best gears are US Gear or the italian made Motive gears not the ones they have made elsewhere.

 

[1969_Mach1 333]

I've used the Italian made Motive gears a couple of times.  Both times they setup easily and were quite.  The 3.50:1 ratio is easy to setup and provide good quite results.  Just stay with one of the two brands barnett468 suggested.  The Italian made  Motive gears are their Performance line, not the blue boxed OEM replacement line.

[smidee 10]

what about strange gears

 

[barnett468 418]

11 hours ago, smidee said:

what about strange gears

 

strange sells italian motive and us gears. i posted a link to them.

[1969_Mach1 333]

To be certain you know what you are getting I'd stick with the brands labeled US Gear or Motive's Performance Gears.  Both are easily available from Summit Racing.

[RPM 1,190]

19 hours ago, Midlife said:

Most interesting.  I have never run across this explanation for gear whine due to common ratios in any of my automotive documentation. 

The "ironic" fact that the 3.70 gears that have no common factor, but tend to make noise kinda proves that the noise has nothing to do with the ratio. So somebody did a little math, made a chart and comes to a conclusion his facts don't support. 

[Midlife 814]

1 minute ago, RPM said:

The "ironic" fact that the 3.70 gears that have no common factor, but tend to make noise kinda proves that the noise has nothing to do with the ratio. So somebody did a little math, made a chart and comes to a conclusion his facts don't support. 

Musta been a politician...

[Mach1 Driver 560]

1 hour ago, Midlife said:

Musta been a politician...

Or (shutter) a journalist

[barnett468 418]

It's simple. The numerically higher the gear ratio, the louder they will be because the faster they are turning. Even a 2.80 ratio whines a little at freeway speeds, however, because it does not whine loudly, and it is insulated from the car by the body and interior and the whine is overpowered by general road noise and engine noise etc, it is impossible to hear unless the gears are bad etc.

[Vicfreg 771]

The ring gear always turns the same amount of RPMs at the same speed, no matter what the gear ratio is, as it is directly connected to the axles, and wheels.  So, at 60 mph, a 4:11 ring gear and a 3:00 ring gear (assuming the same tire outside diameter) turn the same RPM.  One revolution of the wheel/tire = one revolution of the ring gear. 

The gear that does not always turn the same amount of RPMs is the pinion gear, which is connected to your driveshaft and transmission.

Other than mechanical mis-alignment, or bad gear teeth, the issue that causes gear noise are the harmonics caused by the relationship between the pinion and ring gear.   This is the whole "hunting" and "non-hunting" discussion.   There is a lot of engineering literature on this, as well as practical info.   I will say that for most of us, Barnett is right, that it is impossible to hear the gear noise anyway.  

More info here as I am for sure not the expert, just an old engineer who dusted off a text book on this.....

 

http://www.fordmuscleforums.com/all-ford-techboard/565561-9-inch-gear-timing.html