409 guys - need help with rear gear ratio

I need some advice on rear gear ratios. 380 hp 409, TH 350 trans, 26.5" tires, lots of highway miles at 75 mph.

Any advice on the rear??

 

2.73 will get you down the highway but you'll sacrifice in the acceleration department.

 

3.08 was a common ratio. Lots of Pontiac/Olds rear ends around with those gears

 

Well, I've never driven a 409 Chevy, but I'll do some math for a rough estimate.

Gearing to cruise at maximum torque is usually considered a good compromise between response and fuel economy.

A 380hp 409 is spec'd for max torque at 3200rpm.

This works out to a 3.37 ratio for a manual transmission and a 3.20 ratio ***uming 5% torque comverter slippage for an automatic transmission.

 

Keystone?!?!?! You too good to put down Hermosa!??


Have you played with the Richmond gear calc? What I can say is you don't want the 3.89s I had in the T!

 

I have 308 posi with a 390 horse bbc and th400, 75mph doesn't feel too wound up and still smokes 'em at will off the line.

 

what a lot of people don't realize is that a 409 has a big bore and short stroke similar to that of a 302 chevy....my dad ran 4:88's in his 409 55 Chevy, car went high 11s on the street in 68....but seeing that you are going for cruising, I would think a 3.08, 3.23 gear would be great all around...playing with the timing, carb, advance springs etc will help you to tune it to get it where it isn't sluggish off the line yet still is nice on the highway

 

3.08 is a common ratio for Chevys 57-64. Often in 6 automatics (just took one out a couple of weeks ago in a 63 Belair 6/auto). I have never seen anything lower than that in those diffs but they may exist.

 

I run 3:23 posi Olds/Pontiac in my 61' Bel Air 421ci W but also have a Richmond 6 in front of that with 3:27 first gear and .76 OD. and 27" rear tire. 3:36's would probably work best for your application.

 

My 63 SS with a 425 horse 409 and 4 speed cruised nicely with a 3.36 posi and g60-14 pro-trac tires (about 26" tall), also clicked off 13.20's in the 1/4 mile....

 

Every stock Chevy posi I ever saw from that era had 3.36 gears were other ratios available?

 

search: project33.com and when there click on calculators. 350 trans sounds weak for even a stock 409. 400, 700R or manual would be better. yes, low 3's will be OK and give you driveability but, a waste of a big motor.

 

Haha tman. I am going to put Hayward...... Public thank you for the great welcome to the Hills. Thanks.

Thanks for the responses. Looking like somewhere around 3.30.

 

Hayward is closest.

 

3.08 3.36 3.55 3.70 4.11 4.56 from Chevrolet
I have a 3.08 in my 62 Belair 409 5 speed richmond works well with the torque the 09 puts out

 

It'll do OK with a 3.36. But 4.11 or 4.56 are a bunch more fun. Mileage might suffer a bit tho...ya might get 3 mpg like my 62 did.

 

I put a 3.73 in my 61, 421 inch, 409. Wasn't a daily driver, but it was fine for shows from Orange County Raceway to Bakersfield (Fomoso) Raceway, basically the SoCal area.
28" tire.

Ran great, no problems what-so-ever.

Mike

 

This should help -

http://www.summitracing.com/expertadviceandnews/calcsandtools/gear-ratio-calculator

Plug in your tire height, MPH and RPM desired. It ain't pretty with no overdrive at 75 mph.

 

I was thinking 3.36's...

 

I had a 3:36 posi in my 62 Biscayne. Drove great on the street.