Technical Best rear diff ratio?

Hi guys, new to HAMB,
Need some info on rear diff ratio. I am building a '30 model a tudor with a .060 over 392 Hemi.
Running a Ford 9 inch and I am trying to decide whether to run 3:50 or 3:70 rear gears. Rear tires are
30" tall. I will be using a 5spd and the car will be for street use. Any comments or suggestions would be appreciated.
Thanks
Grims

 

With that tall of a tire, I'd run the shorter 3.70 gear.

 

Depends on how you want to drive, cruise and put around, slam gears and go quick and fast, or something in between ?

 

Not sure what the ratio is for your 5 speed, but most are in the .7 range. With the 3.70 rear you will turn about 2000 RPM at 70 and 1600 at 55mph. If you don't have an outragous cam that big Hemi should have plenty on torque to pull the A effortlessly.

 

Your engine has a lot of torque plus your car is very light, it will pull the 3.5 gear easily. I have a 1938 Plymouth pick up with a Mopar 360, tires as tall as yours, 5-speed over drive, 3:23 rear gear, and it pulls it easy, with much less power and much more weight.

 

lots of different 5th gear ratios available for 5 speed transmissions.

I think the last guy, we suggested 4.11 and a 3 speed trans, no overdrive. Or maybe that was just my suggestion.

 

I would go with the 3.70 . I run a 3.73 with 5 speed in a '36 coupe , 27 inch tall tire , .78 od , rpm is a good mix between 55 mph around town and 70 mph on 500 hundred mile runs . carbs don't like to get down in 1800 to 2100 rpm range imo .

 

3.70

Mike

 

Did I miss something? Where did he post what transmission he's running?

 

He just said 5 speed, but obviously not enough info. With overdrive and 30" tires he could run 4:11's no problem.

 

I concur!
Kind of wish I had not chosen the 3.50's in my roadster.
Gotta keep it "on the pipe" ya know.

 

How long is the rope?

 

I have seen 5-speeds with overdrives from 0.89:1 to 0.68:1. That's quite a range. They could be two gears in the same transmission as they are so far apart.

 

I worked this out differently. I have a 6,000rpm red line, and I'll only go up to 60mph in 2nd gear at a red light GP. So put in your rpm, tire size, ****** gear (my 2nd ratio is 1.98), and rear end ratio. Then look at the chart for 60mph. I needed a 3.8 rear. If your 5th gear is .68 put that in for a ****** gear ratio, and it will give you a speed chart for any rear gear you enter. You can even compare three different rear gears at the same time.

http://www.wallaceracing.com/gear-speed.php

 

check sites that have calculators to factor info in to compare results - maybe project33.com

 

Post what 5 speed your running and what 5th gear ratio is (if you have that info ) but 370+ will most likely be your best bet. My bet is you want to run an S-10 trans and rear end. If so 3:73 ---1st choice. Bill

 

With the only choices being 3.50 or 3.70, I personally would run the 3.70.

 

What's the total weight of the car and what are all 5 ratios?

To me, there comes a point where going any lower won't provide anymore dig especially on a lighter car----the wheels will just spin. Now maybe you like that..........

 

Here is a calculator to play with to try different ratios and tire sizes to see what rpm the engine is running with different gears and tires.
The calculator can be used to see the speed and rpm range through each gear to see how the combinations work together. The balance is to get to have an rpm at cruise where the engine is in an efficient place on its power curve. The 3.73+ rear ends help get the rpm up to be efficient & not be a dog.
https://www.teambuick.com/reference/calculators.php

 

In my 30 Steel full fendered weight 2,500 , With a 3:42 with 28- 3/4 tall 7 inch wide Bridgestone tire's ,
6:60-6:70's in 1/8 mile over 100mph . Looking @ maybe going to 3:08 or 2:90

 

Those aren't the only choices.

The choices are:
2.50, 2.80, 2.86, 2.91, 2.94, 3.00, 3.07, 3.10, 3.15, 3.25, 3.33, 3.40, 3.45, 3.50, 3.55, 3.60, 3.70, 3.75, 3.82, 3.98, 4.00, 4.11, 4.22, 4.33,
4.44, 4.50, 4.56, 4.63, 4.71, 4.86, 5.00, 5.14, 5.29, 5.43, 5.5, 5.67, 5.83, 6.00, 6.20, 6.33, 6.50, 6.60, 6.80, 7.00, 7.20, 7.40.

I hear rumors that there was 2.43 ant one point, too.

 

OP !! Make it easy for yourself "" Quick change ""
Like others have said ,it all depends on what you're trying to do and how you want your car to perform

 

3.70 at a minimum. 5 speed.....if 5th is overdrive, then even a 4.10 would be acceptable. Key is what RPM will that early Hemi cruise at.

 

I'm running a 354 in my avatar roadster . I have 14x31 double diamonds on the rear and a 9" 3.70 trak-lok. I have a 3 speed w/overdrive from a 1976 Pacer for a trans. AMC used a Ford full syncro 3 speed adapted to a Lay**** De Normanville overdrive with a .70 percent overdrive ratio. The O/D now is known as Gear vendors . I have about 14k miles on the combination and find it perfect for good take off and great highway cruising. I have a Schneider cam with 288/435 specs and a Holley 670 street avenger. The overdrive is electric controlled by a switch on the dash and I can turn it on or off as I wish at any speed. I don't see any problem with your tire/gear/ 5 speed overdrive.

 

Do the math

RPM x Tyre Diameter [Inches] ÷ Ratio [diff x 5th ] ÷ 336 = MPH

or to calculate it backwards

RPM x Diameter ÷ MPH ÷ 336 ÷ Gearbox Ratio = Diff Ratio

eg: 2000 RPM x 26" ÷ 60 MPH ÷ 336 ÷ 0.70 = 3.68: 1 [Diff Ratio]
or 2000 RPM x 30" ÷ 65 MPH ÷ 336 ÷ 0.70 = 3.92: 1

 

lol you said "Math"

 

I had a 3.00 gear in my morris minor 5.0 ho with aod . It lugged the motor at anything under 55mph.
Bought a Speedway 4.11 chunk and it was a whole new animal. It cruised at 2400 at 65 mph instead of 1400. Still got over 20 mpg.
Go with the 3.70.

 

Gimpy, I meant that those were the OP's only two choices, per his original post. Lol, I have a pile of 9 inch r&p's, from 2.75 to 4.86.

 

I better check on my 2.75, it is from a 31 spline 9 3/8ths.