What is the best rear axle ratio for our cars?

[tpj71mach]

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.

 

[jpaz]

I have a 3.50 gear with a Tru-Trac differential. I really like it but on the freeway it runs around 3200-3300 rpm’s at 70 mph. I have 15 in wheels a C6 auto and I can break the tires loose at 45-50. My 351c is built up pretty good but nothing radical at all. It’s a great combo and if I did more freeway driving I would’ve kept the 3.25 it came with, but I do more street driving around town than anything else.

 

[TommyK]

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.

 

[71coop]

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.

 

[Sheriff41]

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¢

 

[Hemikiller]

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.

 

[tpj71mach]

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.

 

[Hemikiller]

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.

 

[basstrix]

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

 

[Sheriff41]

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.

This.

 

[giantpune]

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.