Rear Gears

[formula46two]

I will be  installing posi and considering the rear gear. The engine makes around 400 horse and is a 4 speed
.  It has 3:42 gear but I would like lower my cruise rpm so what would be a good choice? 3:23?

[Wallington]

If keeping the 4 speed maybe even 3.08, depends how much top end cruising you want to get back. One step might not be enough.

[formula46two]

If keeping the 4 speed maybe even 3.08, depends how much top end cruising you want to get back. One step might not be enough.

That might be a better option. Would like to be close to 2500 rpm at 70 but still have some quickness through the gears

[79GoldnTan]

Yep,3.08 in mine, like it. Cruises nice on the high way and pulls thru the gears just fine. An overdrive would be ideal but only putting about 1000 miles a year what I have is fine.

[b_hill_86]

I put a 3.08 in my 77 4 speed and with a 27” tall tire I’m at around 2700 rpm at 70mph.

[silver78]

Be cautious of the "1st gear" gear ratio in your 4 speed.  If its a muncie with the 2.20 first gear ratio and you change the rear axle gear from the 3.42 to say a 3.08 you're probably gonna be disappointed in the off the line performance.  Here's the math
using a Muncie as the trans.  Change the numbers to match your particular transmissions first gear ratio.

1st gear X rear axle gear = 1st gear ratio
2.20 X 3.42 = 7.52
2.20 X 3.08 = 6.77

Most performance drivelines have an 8.0 or higher 1st gear ratio for quicker acceleration off the line. Example...
2.20 first gear X 3.73 rear gear = 8.20 1st gear ratio

Hope this helps.

[phil400]

I put a 3.08 in my 77 4 speed and with a 27” tall tire I’m at around 2700 rpm at 70mph.

I would have thought with a 3.08 with that tire would have cruised 75mph at 2700, I'm gonna have to check the accuracy of my factory tach.

[79GoldnTan]

I have factory size tires, I think 225/75/15? Sorry got a lot going on in my head with numbers. But at 70 about 2700rpm with factory tach. Don't know how accurate they are but can peg it at 100 in third at around 3500 then hit forth..... how fast is 4200 rpm,4th gear??? Fast enough!

[silver78]

To calculate RPM at a given MPH the formula is:

MPH x gear ratio x 336 divided by tire dia = RPM
 70   x    3.08     x 336     /27                  = 2683 RPM

The stock 225/70-15 tire is 27" in dia.  At 70 MPH the actual RPM is 2683.

I've decided to install a 5-speed overdrive in my TA and keeping the 3.08 rear gear. So, I've been very deep in researching the topic lately.

[silver78]

I have factory size tires, I think 225/75/15? Sorry got a lot going on in my head with numbers. But at 70 about 2700rpm with factory tach. Don't know how accurate they are but can peg it at 100 in third at around 3500 then hit forth..... how fast is 4200 rpm,4th gear??? Fast enough!

You're cruising a 110 MPH at 4200....yup fast enough!

[highway star]

I will be  installing posi and considering the rear gear. The engine makes around 400 horse and is a 4 speed
.  It has 3:42 gear but I would like lower my cruise rpm so what would be a good choice? 3:23?

What posi are ya gonna install? I'm satisfied with the Eaton Posi in my truck. Added Richmond gears & bearing kit.
My '78 T/A has the oem posi diff., still ops good.

[scarebird]

To calculate RPM at a given MPH the formula is:

MPH x gear ratio x 336 divided by tire dia = RPM
 70   x    3.08     x 336     /27                  = 2683 RPM

The stock 225/70-15 tire is 27" in dia.  At 70 MPH the actual RPM is 2683.

I've decided to install a 5-speed overdrive in my TA and keeping the 3.08 rear gear. So, I've been very deep in researching the topic lately.

I have been pretty satisfied with the TKX in my Lemans; 2.93 rear.  235-55-17's

[silver78]

To calculate RPM at a given MPH the formula is:

MPH x gear ratio x 336 divided by tire dia = RPM
 70   x    3.08     x 336     /27                  = 2683 RPM

The stock 225/70-15 tire is 27" in dia.  At 70 MPH the actual RPM is 2683.

I've decided to install a 5-speed overdrive in my TA and keeping the 3.08 rear gear. So, I've been very deep in researching the topic lately.

I have been pretty satisfied with the TKX in my Lemans; 2.93 rear.  235-55-17's

I decided on the TKX 5 spd trans with the 3.27 first gear and .72 overdrive while keeping my 3.08 rear gear.
Calculated 80 MPH will be a reasonable 2200 RPM and first gear will rip = to a Muncie 4 spd with a 4.56 rear gear.
Good thing there's no cup holders in this thing!

OP sorry for the thread highjack...I'll go play by myself now.

[scarebird]

That is the same ratio I am running.  I was getting 2,200 at 70 in OD IIRC.

[tajoe]

I always thought a 3.5 gear to be a perfect all around gear for a Pontiac. But if all you do is hi-way cruising, I guess up-ing it would be justified. Especially considering fuel prices adopted by the current administration. Sad.

[formula46two]

Be cautious of the "1st gear" gear ratio in your 4 speed.  If its a muncie with the 2.20 first gear ratio and you change the rear axle gear from the 3.42 to say a 3.08 you're probably gonna be disappointed in the off the line performance.  Here's the math
using a Muncie as the trans.  Change the numbers to match your particular transmissions first gear ratio.

1st gear X rear axle gear = 1st gear ratio
2.20 X 3.42 = 7.52
2.20 X 3.08 = 6.77

Most performance drivelines have an 8.0 or higher 1st gear ratio for quicker acceleration off the line. Example...
2.20 first gear X 3.73 rear gear = 8.20 1st gear ratio

Hope this helps.

Thanks I will use the formula to make decision. BTW it's a Super T10