Rear gear swap?

[69restomod]

Hey guys I'm brand new to the forum been a fan of the website for awhile and finally joined! Anyway I am wanting to swap gears in my truck from 3.42s to a 3.90 or higher ,I am looking for the best performance and track times I shift at 6500 and will spraying a 150 shot ,I am running a 30 inch tire and was looking for some advice, any help would be greatly appreciated, here's my setup

69 shortbread weighs 3780 with me in it
11-1 383 4 bolt main
Scat forged crank rods and pistons
Comp cams hyd roller cam 242/248 .584/.579 on a 113 lsa
(Bare with me) pro comp 190 cc heads all new Pbm valves and springs
Ported super victor single plane
Holley hp 750
Quick fuel billet 175 fuel pump with -8 an braided fuel line
Sumped fuel cell mounted between the frame rails in rear
Msd pro billet distributor and digital box
Fully built 700r4 hardened output shaft etc.
Edge 3600 lockup converter
12 bolt posi with 3.42s
Ece 4/6 drop
Ece tubular rear trailing arms

[Blue Rat]

With that camshaft. I would not hesitate to go to 3.90's. Especially with a 30" tire.

[69restomod]

Thanks blue rat that was my original thought was to go 3.90s but saw quite a few guys on here running 4.56s with some fast times, but wasnt sure if that would be to much gear and have a hard time hooking with the 700r4s first gear

[Blue Rat]

4.56 would probably be okay for the 1/8 but rough on the 1/4 mile.

[Super73]

Originally Posted by Blue Rat

4.56 would probably be okay for the 1/8 but rough on the 1/4 mile.

I run a 4.56 and 30" tire and still have a little gear left. It all depends on RPM..

[Blue Rat]

Originally Posted by Super73

I run a 4.56 and 30" tire and still have a little gear left. It all depends on RPM..

I was thinking about street driven 4.56's. I guess with a 30" tire it would not be that bad after looking at the numbers. Was also thinking about wheel spin.

[69restomod]

ok cool thanks

[Super73]

What RPM do you spin the motor to? That will ultimately help us figure out the best gear ratio..

[69restomod]

Hey super73 I have my limiter set at 7000 Rpm and I usually shift it At 6500

[Super73]

And what mph do you expect to run?

The needed info for the equation:
RPM
Desired MPH
Gear
Tire Height
Converter slip %


Here is some figuring for you.
7k + 4.88 = 119.0 mph including 7% converter slip
7k + 4.56 = 127.5 mph including 7% converter slip
7k + 4.10 = 141.7 mph including 7% converter slip
7k + 3.90 = 149.0 mph including 7% converter slip

[low&slow]

Originally Posted by Super73

And what mph do you expect to run?

The needed info for the equation:
RPM
Desired MPH
Gear
Tire Height
Converter slip %


Here is some figuring for you.
7k + 4.88 = 119.0 mph including 7% converter slip
7k + 4.56 = 127.5 mph including 7% converter slip
7k + 4.10 = 141.7 mph including 7% converter slip
7k + 3.90 = 149.0 mph including 7% converter slip

I'll defiantly be writing down those examples, but how do you come up with the percentage for converter slip? And what is converter slip?

[69restomod]

I'm thinking around 115 on motor and maybe 125+ on juice and that's being generous. I'm guessing the mphs you posted where in 3rd gear or 1 to 1?

[Super73]

Yes a 1-1 ratio.

[69restomod]

blue rat you are correct it is a street truck and i am considering wheelspin im trying to figure out that happy medium lol thanks super 73 those numbers help out alot

[C20-67_N_MO]

Originally Posted by 69restomod

blue rat you are correct it is a street truck and i am considering wheelspin im trying to figure out that happy medium lol thanks super 73 those numbers help out alot

Dang!

I hope I never meet you on the street for a race then.

My '70 C/10 will have a 383 engine but not near as much cam as what you have stuffed in yours. I live just down the road from you, maybe we can hook up and compare notes on these old trucks.

If you are going 3.90 gears what rearend do you have in your truck?

[Super73]

I am using the % of verter slip that I see for the examples above.

A auto trans always has some form of slip to it unless it has a lock up converter and that converter is locked. I don't know a good way to explain it, but I will try.

Think about a manual transmission for a second. The input shaft is connected up via splines in the clutch. Once the clutch is out it is a direct drive. Inside the trans, gears are mechanically locked in place to one another and can not slip or jump.

An auto trans on the other hand uses a hydraulic fluid pump to ultimately move the vehicle. The converter is bolted to the flywheel with the input shaft of the transmission sitting in splines inside the converter. There is a snub on the converter that drives the fluid pump. If I am correct, as RPM increases centrifugal force then mates clutches together in the converter turning the input shaft of the transmission. These clutches never fully lock like a manual transmission. There is a lot more slip on the starting line than the finish line.

I am sure a tranny guy could explain that way better!!


If you know your trap speed and what RPM you cross the line, it is easy to do the math to figure out slip %. The info you need to figure out converter slip:

RPM
Gear
Roll out (which is tire height * 3.14)
MPH through traps
RPM through traps

The equation is to give you what MPH should be with 0% slip is (IE Manual trans):
RPM / Gear * Roll out / 12 * 60 / 5280

But since there is slip, you take your actual MPH and divide it buy the out put of that equation, then subtract that from 1. Here is mine for example:

6950 / 4.56 * (30*3.14) / 12 * 60 / 5280 = 136mph
This was a motor pass and actual trap speed was 127.
127/136 = .933
1 - 0.933 = .067 which is 6.7%

7250 / 4.56 * (30*3.14) / 12 * 60 / 5280 = 142mph
This was a pass on the 100 hit and actual trap speed was 132
132/142 = .929
1 - 0.929 = .070 which is 7%


Keep in mind, I am on a radial tire. If I was on a slick, you would have to also account for tire growth.

[low&slow]

How do you figure out tire growth?

I'm running goodyear eagle slicks

[Super73]

Low&Slow.. Figuring out slick growth is hard. I don't have a real answer for you except ask the MFG..