What is the best rear axle ratio for our cars?

I was just replying to a thread from jscott about a noise he is hearing that sounds just like the noise I am starting to hear. It seems the general consensus in that thread is that it is probably the differential failing, which I expected from my car but was trying to get the rest of the restoration done first. You know, make it shiny first then fix the problems, LOL. As it looks like the rear differential is now going to need to be replaced over this winter, I have a question about the best gear ratio to give a good balance of off the line and highway performance.
I have a 351C with an FMX and a 9-inch rear end. The car came with a 3.0 open case rear differential, and I am going to upgrade to a limited slip of some sort, TBD. With the 3.0 ratio the car ran at 75 MPH quite easily and the RPMs remained low, and the temperature stayed cool after over an hour of sustained highway speed. That is the great part of the tall gear, but I would like to get a little more gear to help the off the line performance. I was originally leaning toward a 3.5 gear ratio and a clutch type of limited slip unit.
I know there are a lot of people on this forum that are way more mechanically and performance knowledgeable especially in regard to Fords than I am. I am hoping to get some guidance on this. My go-to ratio with my GM cars is a 3:42 limited slip in a 12-bolt rear differential. That has always given me a good balance of off the line and highway speeds. That is with a GM turbo 400 transmission though, and I do not know what best rear differential ratio for an FMX is. I would think that the high gear in the transmissions would be totally different between the FMX and 400 transmissions and would change the rear differential ratio needs.
Another factor is what type of limited slip do you think is best? My car will be just a driver that will never really be raced, with the exception of the occasional burnout. I was leaning toward a clutch type over the gear type for my needs but would like to get opinions from you guys who know the pros and cons of each. Also, is there any way to determine if the axles are 28 or 31 splines without disassembly? I was hoping there was some standard where all open case diffs were 28 spline or something like that. Or maybe a stamping number on the hup of the axle? I would prefer to order the parts before I disassemble if possible. Any assistance and information would be greatly appreciated, thanks.

The gear ratios are almost identical between the FMX and T400.

A stock trac-loc is more than adequate for street use.

For a stock or near stock engine, gear ratio is largely a matter of personal preference.

You can use any number of online calculators to determine your cruising rpm if you know your rear tire diameter and the final drive ratio of your trans which in this case is 1.00/1.

If you do a lot of highway driving a 3.25 is usually a good compromise.

I've run both a 2.79:1 and a 3.5:1 truetrac in my Mach 1. I do a lot of highway driving, so the 2.79 is actually my preference eof the 2. I also built my 351c to have a pretty broad torque curve so it has plenty for me off the line for street driving. The 3.5 was better off the line and more fun, now that I'm spoiled to modern transmissions & gearing, it does wear on me when on the highway, especially on a trip more than a half hour long, which is common for me. If our speed limits were still at 55 or so, I would feel differently. A lot of roads around here are between 60-75 so that is a factor to think about.

For my taste, I would like to go to about a 3: 1, but definitely no more than a 3.25:1.

Another thing to think about, and for me this is more of an issue than the RPMs, is to make sure your replacement diff doesn't whine. Even a small whine at highway speeds can become annoying, and that is common with rebuilds - so find a good, very experienced supplier/builder.

I would stay in the 3.00 to 3.70 range depending on your intended use of the car. 3.25 gears are good for all-around cruiser/touring with a 4-speed. I run 3.89 gears with a 5-speed and would probably switch to 3.70 if I change them in the future. I
Just my 0.02¢

Final drive on any non-overdrive automatic is going to be 1:1, so for all intents and purposes, a TH400 = FMX = C6 etc. Converter slip is really the only variable involved.

You need to take into account tire diameter and gear ratio. A 26" tire with a 3.40 rear gear was an excellent all around setup with a 302 / C-4 combination I ran way back. Worked great with a toplaoder 4 peed as well.

Thank you for all the info, I really appreciate how many really knowledgeable people are willing to help with advice on this site. I was pretty sure the 400's were 1:1 but TBH I had no idea what the FMX ratio would be.

I have 245/60R15 Goodrich T/A's on the car which have a 26.6" diameter. With the 3.0:1 ratio I am around 2700 at 70-75 MPH which is great on the highway for me. The car has good off the line performance, the engine has plenty of power and torque (437HP, 451torque at crank) and have a low stall speed converter. TBH, if the differential wasn't an open case and wasn't failing, I probably wouldn't change it. I would like to get a little more off the line though if I can without shooting myself in the foot on highway speeds. I feel that if I am going to replace the center section and axle bearings and seals, I should change ratios or forever hold my peace. I like the 3:42 ratio in my other cars but when I look at Speedway, for the 9-inch center sections they list 3.0, 3.25, 3.50, 3.70, 3.89 and so on. I am concerned that the small jump from 3.42 to 3.50 might be that proverbial straw that broke the camel's back, maybe the 3.25 would be the safer bet. It looks like a few of you like the 3.25:1 ratio, I think that would give me a little more off the line without hurting highway drivability too much.

Pardon my ignorance but is the Trac-Loc a clutch type posi or a gear type and which would be the better option? Is True trac a different type of posi unit or are these just name brands and are similar. I have always heard people referring to limited slips as clutch type, gear type or spools. I know I do not need a partial or full spool for what the car can do and what I want to do with it. That left me with the question of which is better for my needs, clutch or gear type unit.

I installed a 3.89 posi in my 73 in high school and it screamed at highway speeds, so I learned at 17 how poorly thought out that decision was. It took a long time to save the money for that set up, so I was pretty much stuck with it. To make it worse the 3.89 ratio decision was after a friend talked me out of the 4:11 ratio I was originally wanted to get at that time, haha.

If you're happy with the 3.00 rear, you can get one built with a limited slip, or go to a 3.25 for a little extra oompf. I bought my 4.11 rear from Quick Performance and it's very quiet. Alan at https://8and9inchfordrears.com/ also comes highly recommended.

I would say the Truetrac would be a very good choice for your application. The Trac Lok is fine, but you have to mess with friction modifier and the clutch plates will wear out, eventually.

Here's a good description of the 3 major types of limited slip differentials: https://www.currieenterprises.com/differential-options-explained

Hemikiller said:

If you're happy with the 3.00 rear, you can get one built with a limited slip, or go to a 3.25 for a little extra oompf. I bought my 4.11 rear from Quick Performance and it's very quiet. Alan at https://8and9inchfordrears.com/ also comes highly recommended.

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This.

With a non-overdrive transmission and our stock 26" tall tires, I wouldn't go any numerically higher than like 3.25 for the rear end. Especially with an automatic transmission that will also have torque converter slip of like 7-10%.

For the type of limited slip, I like the true-trak. Its less maintenance. I have one in a GM 12 bolt I put together for a farm truck and another one in a ford 9" for drag racing. So far, no complaints.

I have 1 car with a 3.73 on a true-trak, TH400, an 26" tires. 45mph is 2350RPM. 60mph is over 3000rpm. Its not something you'd want to take on the highway for more than a few miles. This car is on my short list for getting a transmission swap to something with an overdrive.

Hmm, it all depends on where you see yourself going. You are in the polish stage of a polish then repair build so you really need to consider where you are going to end up when you are 100% complete. Sounds like you have some decent performance from the Cleveland so I would guess you are not going to be touching that anytime soon. For us FMX guys who really want the best of both worlds there is always a tranny swap. There are guys on here who can walk you through installing a 4 speed auto tranny. AOD or 4R70W are the two best choices I came across when I looked. You end up with the same spread on the bottom 3 gears and 0.67:1 on the top gear. The AOD is the best choice for someone who doesn't want to deal with an electric transmission and the 4R70W is great if you want to tune or mess around with shift points. The cost of a 4R70W is a bit higher since you need an electronic shifter but there are companies out there making it easy or you can go crazy and pull an EEC-V controller out of an existing Ford into the equation to control and save a ton of money (be leary of the donor car, most 4R70W's are not Essex bellhouse patterns like SBF's the best is a 2003/4 Mustang V6 as donor and an EEC-V from a 2003/4 Maurader/Crown Vic/Lincoln Town car, just in case your rabbit hole leads you to an EFI conversion one day, same fire order as a Cleveland). Like I said, it's all where you want to end up. If your goal is to NOT sacrifice fun in the city for pain on the hi way, then an AOD is a great idea. You can go ahead with the 3.5:1 to the 3.7:1 without being permanently locked in to 3000+ rpm on the hi way. Your car = your plan. Just be forewarned, once you set your foot onto the restomod path forever will it dominate your destiny.

AcesArneson said:

Hmm, it all depends on where you see yourself going. You are in the polish stage of a polish then repair build so you really need to consider where you are going to end up when you are 100% complete. Sounds like you have some decent performance from the Cleveland so I would guess you are not going to be touching that anytime soon. For us FMX guys who really want the best of both worlds there is always a tranny swap. There are guys on here who can walk you through installing a 4 speed auto tranny. AOD or 4R70W are the two best choices I came across when I looked. You end up with the same spread on the bottom 3 gears and 0.67:1 on the top gear. The AOD is the best choice for someone who doesn't want to deal with an electric transmission and the 4R70W is great if you want to tune or mess around with shift points. The cost of a 4R70W is a bit higher since you need an electronic shifter but there are companies out there making it easy or you can go crazy and pull an EEC-V controller out of an existing Ford into the equation to control and save a ton of money (be leary of the donor car, most 4R70W's are not Essex bellhouse patterns like SBF's the best is a 2003/4 Mustang V6 as donor and an EEC-V from a 2003/4 Maurader/Crown Vic/Lincoln Town car, just in case your rabbit hole leads you to an EFI conversion one day, same fire order as a Cleveland). Like I said, it's all where you want to end up. If your goal is to NOT sacrifice fun in the city for pain on the hi way, then an AOD is a great idea. You can go ahead with the 3.5:1 to the 3.7:1 without being permanently locked in to 3000+ rpm on the hi way. Your car = your plan. Just be forewarned, once you set your foot onto the restomod path forever will it dominate your destiny.

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In addition to the overdrive, the AOD and 4R70W have lockup converters. So its not just the 30% reduction in engine RPM. You also get the 7+% back from the TC slippage.

giantpune said:

In addition to the overdrive, the AOD and 4R70W have lockup converters. So its not just the 30% reduction in engine RPM. You also get the 7+% back from the TC slippage.

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Also need to consider the difference in parasitic power loss. the FMX is the star at 25hp and the AOD has been called in around 40hp. 7% of 437 hp is 30.6hp so the lock up will still lead to more power with an AOD even though it is slightly more parasitic. Interesting!

AcesArneson said:

Also need to consider the difference in parasitic power loss. the FMX is the star at 25hp and the AOD has been called in around 40hp. 7% of 437 hp is 30.6hp so the lock up will still lead to more power with an AOD even though it is slightly more parasitic. Interesting!

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The torque converter slippage is not taking away from the horsepower. It just never fully locks up. So 1 revolution from the engine's crankshaft does not equal 1 revolution of the transmission's input shaft in an older transmission like the c4, c6, fmx, and turbo 350/400. What it means is your engine has to turn faster at a given MPH as compared to a manual transmission or an automatic that does use a lockup converter.

The AOD used a mechanical lockup converter with a 2-piece transmission input shaft. Those were a failure point if you tried to put too much power through them. The 4R70W and GM's 700r4 use an electronic lockup. You apply 12v to a wire and it closes a clutch inside the torque converter. It gives you more flexibility for when to lock and unlock it. In my farm truck, its a switch on the dash. I can be driving along and feel the transmission shift 1, 2, 3, and overdrive. When I hit the switch to lock it up, it feels like there's yet another gear it hops into.

I am a little late to the party. But, our Mach 1 had its original 2.79:1 ratio rear axle gears replaced by the prior owner with a 9" housing and 3.5:1 TractionLok rear axle gear set. I have come to feel that the 3.5:1 ratio is about the lowest ratio I would want to go with a street driven vehicle, and at that the engine RPM as just under 2,900 at 60 MPH. It was tolerable, but still far higher than I am used to with our other First Generation pony cars. So, in a bid to reduce the engine RPM when cruising, while retaining the lower end performance of the 3.5:1 gears, we opted to replace the original c4 with an AOD. I have a suite of YouTube videos, to include videos of the actual replacement itself, under my YouTube channel (Gilbert Hale). In the description of some of those videos I have links to various vendoeds and the parts list used. I am now spinning the engine at around 1,900 rPM at 60 MPH, and have recaptured the upper end performance as the street/strip built engine has plenty of torque to push through the air resistance at triple digit speeds. The OverDrive has provided us with the best of both worlds. I am veyr glad I did it. For that matter, I am betting the AOD would have also nicely tamed even a 4.11:1 ratio gear set, if I had them. I am a happy camper.

mrgmhale said:

I am a little late to the party. But, our Mach 1 had its original 2.79:1 ratio rear axle gears replaced by the prior owner with a 9" housing and 3.5:1 TractionLok rear axle gear set. I have come to feel that the 3.5:1 ratio is about the lowest ratio I would want to go with a street driven vehicle, and at that the engine RPM as just under 2,900 at 60 MPH. It was tolerable, but still far higher than I am used to with our other First Generation pony cars. So, in a bid to reduce the engine RPM when cruising, while retaining the lower end performance of the 3.5:1 gears, we opted to replace the original c4 with an AOD. I have a suite of YouTube videos, to include videos of the actual replacement itself, under my YouTube channel (Gilbert Hale). In the description of some of those videos I have links to various vendoeds and the parts list used. I am now spinning the engine at around 1,900 rPM at 60 MPH, and have recaptured the upper end performance as the street/strip built engine has plenty of torque to push through the air resistance at triple digit speeds. The OverDrive has provided us with the best of both worlds. I am veyr glad I did it. For that matter, I am betting the AOD would have also nicely tamed even a 4.11:1 ratio gear set, if I had them. I am a happy camper.

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I often think I wish I had gone with an overdrive transmission instead of rebuilding the FMX as I was building my car. I was new to the Ford world at the time and didn't realize the bell housings are the same and the AOD's would work with a Cleveland. This was before I was told about this forum, I was pretty much flying blind at that point. I plan to upgrade to overdrive down the road, I am just hoping to get some use out of the FMX I already spent good money on rebuilding, haha.

tpj71mach said:

I often think I wish I had gone with an overdrive transmission instead of rebuilding the FMX as I was building my car. I was new to the Ford world at the time and didn't realize the bell housings are the same and the AOD's would work with a Cleveland. This was before I was told about this forum, I was pretty much flying blind at that point. I plan to upgrade to overdrive down the road, I am just hoping to get some use out of the FMX I already spent good money on rebuilding, haha.

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Unless you plan to do a lot of road trips in your Mach 1, I'd recommend just staying with your FMX. The FMX is a good trans and should last you. I had an AOD in my 72 for a couple decades and it was nice but I'm not sure it was worth the expense and effort. You can buy a lot of gas for the dollars you'll spend on the conversion. I've now switched to a T5Z and the fun factor is definitely worth it!

According to spicer’s rpm calculator with an automatic and in overdrive
3.0 @ 70mph = 2724 rpm, od = 1907
3.25@70mph = 2951 rpm, od = 2065
3.5 @ 70mph = 3178 rpm, od = 2224
3.73@70mph = 3386 rpm, od = 2371

Hope this helps.

I am running 373 and really like them around town. The calculator CJ posted above is pretty spot on for my Rpm’s with my c4. Eventually I will go with a 4r70w. On long road trips you’re a bit limited by the 373s. But I’m usually just cruising around town. And I love them for that.