Thread: Gear ratio question

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I have experience working on every part of a classic but what I don't know I will ask! I recently acquired 2 9in rear ends, one with 3:73? and the other with 2:75. I need to choose the right one to put under my 46 bus. coupe. I want a descent driver that I can drive into Austin about 35 miles from home on the hwy but I hope to still have some pep. In your much more knowledgeable opinion which would you choose and why? I can rebuild a pont 316, 287, olds 303 ect but never learned gear ratios. Plz help and thx. Next q will be what the best way to attach this thing to the existing frame is.

6skullz

Given the choice between the two, install the 3.73's and throw the 2.75's on your scrap heap. Of course it makes a big difference what your final drive ratio is in your tranny, and what diameter tire you're running in the back, but I'd be willing to bet that the 2.75's will be anemic unless there's something really, really funky going on....

What engine are you using, and what transmission? Engine power and transmission ratios make a difference.

sbc 350 with standard bolt on upgrades, mild cam ect. I'm building it around 350 horse so not real beefy, I'm a budget builder so I kind of have to make do with what I have or can afford which leads me to...I have a fresh rebuilt 700r and then theres the 4 speed Saginaw but I'm going with the 700. Cars I've done in the past were already together and I didn't rod them just got them running so this is new to me and I thank you for the quick responses.

If you have 350 HP and a 700R4, I would use the 3.73 gears. The 3.06 first gear in the R4 gives you plenty of boot off the line, and the 0.70 overdrive is essentially the equivalent of 2.75 gears on the top end. Besides, that full-fendered '46 is a bit heavy. Besides, if you used the 2.75's and the R4, you'd have the equivalent of a 1.9 rear gear. That would be hard to pull. Final note - make SURE or double SURE the R4 is adequately cooled. That means a heat exchanger tank in the radiator and an external cooler. It's also best if the R4 is wired to make sure that the converter locks up in overdrive and the lockup releases with the application of the brakes. R4's build heat quickly in overdrive if not locked. You can find a kit a Jegs or Summit - or about anywhere else. I installed a switch on min so I can lock up in any gear.

TCI 376600 TCI 200-4R / 700R4 Torque Converter Lockup Kit - Free Shipping on All Orders @ JEGS

Differential rule of thumb:
The higher the number the more torque,
The lower the number the more economy

A great combination is an overdrive transmission and a higher numeric rear end. Most
newer cars have an overdrive transmission to improve fuel economy. even the old T5s have about a 15% overdrive in 5th gear

Awesome info, thx for the lesson. Now I have something to research today. Thinking about mocking up the coupe while its out, play with stances...if I do I'll snap shots

BTW, bear in mind that the "kickdown" on a 700R4 is not really a "kickdown." It's a throttle valve cable system, AKA TV cable. Driving with improper adjustment of that cable is a death sentence for your transmission. Eventually, you'll want to read the following article - or one like it. Bowtie Overdrives | TV Made EZ

If you want to play with the numbers, Cruise RPM = (mph x Gear Ratio x 336 x OD Ratio)/Tire Diameter in inches. For example, if you're looking at 75mph on the highway, (75 x 3.73 x 336 x 0.70) divided by say 28" tells you that you'll be cranking 2,349.9 rpm, or 2350 which is pretty good. If you run a 29" tall tire you'll drop to 2,269 rpm, even better for economy.
Now if you drop in that set of 2.75's in the back, with that 28" tire and 75mph your cruise rpm will drop to 1,733 rpm which may be a bit below your torque curve on the 350.

Thx Jack, Since I have really only been a remove, repair, replace kind of car guy little tips like that are what I know I'll need to have a successful build. Any advice on installing the 9in? I don't have the skill level to just do it, I'll have to fit it measure, move it, measure again but that's why I'm doing it, time to learn more. Also I want to put disc brakes up front and lower the nose quite a bit, other than drop spindle kits are there other ways to lower it?

Roger, I appreciate that formula. Over my head but thx. I know I can figure it out. I have a fairly complex job but all I really do is read prints to install under ground utilities. I follow directions, this I have to figure out. Pretty cool, I hope I don't lose interest, I am a little nervous about messing something up bad, there is very little budget for this build.

Do you intend to use the stock style leaf springs, or install something more exotic like coil-over shocks? BTW, are you a directional driller? I see you are in Austin, TX. I used to work for Southern Union and designed/supervised hundreds and hundreds of miles of distribution piping & services, river bores, bridge crossings, transmission lines, regulator and measurement stations, etc., etc.

Honestly Jack I don't have an answer... If someone could explain to me why one would be better than the other, or if it even matters then I might come up with semi educated answer instead of openly admitting my lack of real car knowledge by having to answer "I don't have an answer"! I'm a Superintendent with an underground utilities installation company. Site work. Its a fun hustle, you know 80-90 phone calls made and received by noon, Precons, sub-contractor meetings, permits, submittals, GCs, PMs, 5 crews meaning 5 Foremen and 35 operators and laborers who no hable ingles. Sometimes I'm so stretched I feel like I lose track of where I am on jobs and at the end of the day I get a "good job" from the boss....every day is a trip that never seems to end! Oh, and my boss is 10 years my junior but he's sharp as a tack and mean as hell, we make a great team.

On your rear suspension, a leaf spring set up might be easier for you to set up and cost is cheaper too if you search around for a pair of springs you can fit under it. Or you can buy one from these guys and bolt it in.

1941-1948 Ford Car - Chassis Engineering Inc.

Another option is 4 link, or triangulated 4 link which does not require a pan hard bar. These are a little more difficult to set up and you will need to run coil overs, air bags with them. The coil overs or air bags will probably give you the best ride, but the bolt on leaf kit isn't a bad way to go for a someone who isn't a fab guy.