So although I don't really plan to do anything w/my 305 in my Firebird, I figured one mod I should at least do in the meantime is swap in a posi rear, and upgrade the gear ratio.

Right now, it has an open differential rear, with a 3.08 ratio.

What are my plans w/the car? Eventually, I'd like to do some kind of engine/trans swap, whether it be an L98, LT-1, or LS1, and a stronger T-5, or T-56. You get the idea. I just think a swap is more economical than trying to get my power goals reliably w/a 305 TBI and T-5. Figure that the "goal" HP/range is around 300-350 or so at the crank, maybe up to 400 eventually.

Figured I'd explain all that, as I've been told more aggressive gear ratios, like 3.73, 4.11, etc. are probably your best bet for strict performance, at the expense of streetability/fuel economy. This car I mainly just want to use as a nice weather/vacation trip/occasional strip visit car. As it stands, 3.08's only have me @ 1500 RPM's going 65 on the freeway.

I'm thinking 3.42's would be a good choice for what I want, what do you all think?

 

It all depends on what you want to do with the car and your combination. 3.42 gears are a good all around gear. I would leave your gears alone for now until you decide on your motor and trans combination. Then you could pic the right gears for your combination. I think you would be much more satisfied then.

 

+1

I like 3.73s, but if you do go with a t-56, I would recommend 4.10s.

 

Hey 3rdgen, I don't mean to high jack the threat but I read the sig in your profile and it says that you have the lt4 hot cam. I was thinking about getting that cam in my car. How does it perform and what gears do you think fit best with it?

 

3rd gen, just out of curiosity, what makes you recommend 4.10's on the T-56?

So, just to make sure I understand the whole gear ratio thing correctly, higher gearing (lower numerically) = better fuel economy, less performance, and lower gearing (higher numerically) = faster, but lower fuel economy, correct? And what kind of engine RPM's can I expect at freeway speeds top gear with 3.42's, 3.73's, 4.10's? All info appreciated!

 

4.10s work great with the t56 due to the high (low numericall) overdrive of the 6 speed trans versus the T-5 (5 speed)

You are basically correct on gear ratios.

Here is a good calculator to give different rpms at different speeds with different ratios.

http://www.corral.net/tech/gearcalc.html

I really like it. I have been having tons of transmission problems (mostly with people, not the parts). I like the 3.73s. Here is a write up I done on it. What heads do you have?

https://www.thirdgen.org/forums/tech...ortec-lt4.html

Hope that helps some

 

Thanks for the information. My engine is bone stock, with the cast iron heads. I want to upgrade to afr 180 heads when i swap the cam. I have a bowtie overdrives level 2 700r4, with a 2200rpm stall.

Will the cam require tuning? Is it emissions legal, I live in california so I have to deal with that.

 

If you don't plan on getting the t-56 soon then I would stick with the 3.42 gears but if you do, 3.70 are a good all around gear for it.

 

Wow, that's a pretty rad ratio/RPM calculator there, 3rdgen, thanks!

But I still have one question. If my RPM's are slightly lower than what the calculator suggests, does that mean something might be wrong?

And based on what I've been told here, 3.42 should be good for what I want w/a 5-speed, but 3.73 or 4.10 would be better choices for the 6 speed?

I sure am having a tough time deciding if I want to go the TKO 500/600 or the T-56 route, that's for sure! I will be stuck with my WC T-5 for a while though.

 

the calculator is correct. The inaccuracy will come from tach readings and the actual tire diameter. Someone might say a 25" tire but in reality it maybe 24.75 or something like that. And the tach could be off. But mathmatically, the calculator is dead on.

3.42 is a great all around gear. So is the 3.73. There is hardly any noticeable difference that you can feel in the seat of your pants between the two. 3.42/3.73 for the 5 speed, 3.73/4.10 for the 6 speed.

Personally I would go 3.73 with the 5 and 4.10 with the 6.

You will have to do a search for more information on tuning. Im carb so I didnt have any of those problems. Have you upgraded your factory intake? You will need to do that to get the most performance out of cam/head combo. Also, you might want to look into a couple of Comp's Cams. The LT4 hotcam is a great cam, but with the afr heads, you might be able to find a cam that will work better.

 

Yeah thats what I was thinking. I am going to keep the long tube runner set up and go with tpis runners and intake.

 

I just did a rearend swap in my car from an 87 iroc, and i went from 2.73 to 3.23. To be honest, i didnt notice much of a difference. Defiantly not worth the 10 hours of work lol. If i were to do it again i would go with 3.73s

 

What is the best all around gear ratio for a car thats mostly on the street. I only have 2.77 gears and I was thinking about changing them to 3.27 or 3.45 gears.

 

3.45

 

3.42 or 3.73. between the two you wont feel much of a difference, but if you're coming from 2.xx you'll feel it.

anything more than 4-4.11 sounds a bit overkill if you drive it on the street often, especially highway

 

Most of my driving is done on the street and highway so I don't want to loose that.

I am trying to pick a gear set that will match the parts that I have planned. I want to get afr 180cc heads, tpis long tube runners and manifold. I haven't decided on a cam yet.

 

the most common are the 3.42 and 3.73. anything in between really is fine for normal driving + the beatings

 

Yup, definatly dont go with anything below a 3.42, or you will be disappointed, like me lol. But the only reason i went with 3.23s was because i got a really good deal on it. I did notice somewhat of a difference though, it just wasnt worth all the work