Thirdgens came with 2 5th gear ratios...
The 3.42 cars (mine) have a .75
The 3.08 rear end cars got a fuel economy .63 gear.... Better highway mileage...

Since I will probably do more highway driving than drag racing... I'm leaning towards the smaller gear...

My choices now are a .64 or .59...
I calculated @ 70 mph 1758 rpm (with my rear end, tire size, etc...)
The .64 would put rpm @ 1907...

I'm leaning to having the lower .59 for better fuel economy...

I can get to 120 mph with the 4th 1:1 gear, rpm is 5107

.59 or .64 5th gear ratio?

What are your thoughts for or against?

Car is TPI Street car, plan to Turbo in the future...

Do you drag race your T5, are you ever going into 5th?

 

I wouldn't do that.

One time I accidentally put a .63 out of some grocery-cart LG4 or L03 core I had got, into my own. I HATED it. Always seemed like too much of a step, like it needed a 4½th gear; kinda like the step going from 1st to 2nd in a 700. Even with the weenie stock gears in my car, which aren't quite as weenie as your 3.42, but almost; 3.73.

I can't imagine how it could possibly be fun to drive a 3500 lb car with nothing but a 305 and that steep of a gear. You'd probably never even use it after about the 3rd time you put it in it and discovered that the car would go faster with less throttle in 4th.

 

"faster with less throttle in 4th." Because of the wind drag?

I thought 1 through 4th would be my fun gears, and 5th my highway / good mileage gear... Kinda like a T56 without 5...??

 

0.73 isn't tall enough for a cammed L69 with 3.73 rear and 26" tires if you're after running upstairs.

My take is that the 0.63 is the sweet spot for that combo. Adding 0.59 5th to a 3.42 rear car isn't going to be ideal, though it will be tractable.

A LTR TPI intake is just going to ruin your overall powerband if you're going for top end and spending your blood, sweat, tears, and dollars turboing one is a chase in futility.

It stands true however, that torque does make up for errors in gearing a car. Things like 700-R4 1-3 ratios with truck-like rpm dropoff are "ok" if you have power. There are simply better options.

 

You can overdrive a transmission too much and fuel mileage will get worse instead of better. I would go with something that will keep you close to 2000 RPMs at highway speed.

 

I'm trying to understand all this colorful language, grocery kart, running up stairs... etc...

I will rarely drag race the car...
I would like 5th to be a good gear for highway
In town, it's 45, I'll probably do 50 (80% of the other drivers are doing 55)...
I'm not looking for high end HP / speed...
I read another post of a guy with a T56 saying how he was doing 80mph with a .5 ratio and just under 2k rpm...

There actually isn't that much RPM difference between the .59 and the .64... I don't think there are any other choices, there is a .79 for road racing, but I'm not doing that either... may autocross and I'm sure I will not be leaving or maybe even getting into 4th gear....

"Turboing a tpi a chase in futility"?
How about TurboedTPI, old stats..
Boosted and intercooled L98 with 749 ecm
10.72@129-1.5 60ft(new best time Nov/2010)-570rwhp@13lbs on pump gas. Est. 600+rwhp@16lbs on 100 octane.
Featured in GMHTP mag July 07

So big gear head, you would .64 over the .59...

 

I would, but this is just my opinion.

 

If you need a link for a good gear calculator, let me know. You can input your overdrive gear, your rear end, and your ture size and see exactly the difference between the two gears. Pick which ever one us right at or slightly below 2000 at 70 mph

 

I've been using a calculator I found online...
I think the .64 would be better, going from a .75...
It's still more "fuel efficient" than the .75 and not as drastic as .59...
I don't think I have any other choices (I'll check), but I for sure do not want to go bigger and pay for a .79...

 

Yeah, 0.63 is the way to go then.

Don't be butthurt. Just get away from the best looking, worst performing intake known to cro-80s man.

 

Combining TPI, turbocharging, and a tall OD is a good plan, if you don't over-cam, don't get 200 cc heads, and let the turbo do its job.
Sofa needing a 4.5th gear is a simple mismatch of cruise RPM and engine parts, unless the tune was way off. Not just a little off. But I think if Sofa is honest, it was probably a mismatch, not the tune.
My own decades of experience say nothing less than a stock 454 wants anything less than 1800 RPM at 65 MPH, though the LS1 tolerates it better than any 305.
I've been part of 8 separate instances where changing from 2.73:1 gears to 3.08:1 gears helped MPG. All 8 involved a 700R-4, or a 4L60E, which is the same thing only computer controlled.
Less RPM isn't always better, even if it doesn't seem to be lugging. Sure, you'll find it with these new 6-speed automatics, but then they're always in fifth anyway.

 

I have a 3.73 rear and I changed to a .63 from the .72 I love it. I run the expressway fairly often and it was a great change.

 

2004 gto. ls1, ported 241 heads, boltons, stock ls6 cam. t56 trans, 3.45 gear.
averaged 31mpg from Charlotte to Maryland. went over 500 miles on the tank of fuel. car dynoed 430 at the wheels.
6th gear ratio is .57:1. you can do the math for the overall ratio....