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.