[tooski]

After searching for about an hour I couldn't quite find what I needed.

I am going to swap my 4.11's for a more highway/street friendly gear. Considering 3.08/3.23/3.42 (max). 275/60-15 tire (~28").
Car is a 75 Formula with weight of 3600/3700 lbs. Engine is 433 10:1 with 230/240 cam.
I was ok with 3.08's in my 68 Firebird (about 31/3200 lbs). 406 10:1, 230/240 cam. 255/60-15 tire (~26").
Suggestions?

[JLMounce]

Had a set of 3.42's in my wife's 71 Chevelle that had a 27" tire package on it. Stock GM crate 350. With the quietish exhaust on that car it actually did very well on the highway. It had a tight factory style stall converter and at 70mph I want to say it was doing around 2800 rpm.

With your slightly lighter car with a lot more torque, I would probably target the 3.23 myself as a compromise between performance and highway cruisability. That should provide a similar experience to the 3.08's in your 68 firebird with the smaller tire.

I have 3.31's in my 69 firebird that I had with the factory th400. It was still a bit more rpm than my liking compared to my previous 3.73's. Definitely better, but still not ideal for highway cruising with a loud exhaust and short 26" tires. The no compromise solution is obviously an overdrive transmission. That's quite a bit more than a gear change though.

[steve25]

These two charts might help some.

[chuckies76ta]

Be good starting point to know what transmission your running. Also, what rpm does the engine like to cruise at? What are the highway speed limits where you live?

[VCho455]

Here's one of the best calculators to help you make a decision.

https://www.blocklayer.com/rpm-gear

Personally with that tire size I would run 3.55's every time. Unless you have an overdrive. I have a Gearvendor with the same tires in my Burban and I run 3.73's in it.

[tooski]

Originally Posted by chuckies76ta

Be good starting point to know what transmission your running. Also, what rpm does the engine like to cruise at? What are the highway speed limits where you live?

S/S built TH350, and stockish TH400 are available. Engine not built yet. Country highway 90kph,that's 55mph, Very seldom to see anyone at that speed. Mostly 100 to 110 kph (60 to 70 mph). 105/65 will generally avoid interaction with the police.

[PONTIAC LARRY]

3.23's FTW

[Elarson]

I used to have a street-driven '77 T/A that came with 3.23 gears. They were great for mixed driving.

Your positive experience with 3.08 and 26" tire equates to 3.31 with 28" tire.

3.73 was much better for stoplight racing.......so I've heard


Eric

[Scarebird]

2.93/3.08's for all around use, especially with a torquey motor.

[Lemans64]

What rpm is gonna make the engine happy? Use that rpm and calculator with tire size to find the right rpm.
Use a rpm calculator of google.

[grivera]

3.23s would be perfect with a 28” tire

[JLBIII]

I switched from 2.56 to 3.36 gears with a 26" tire in my 68 Firebird with TH400 trans. It felt like I was in 2nd gear on the highway compared to the 2.56 gears. Now running 3.91 gears with a 2004R trans.

[srmmmm]

I am running 3.23 with a 235/55R17 tire, 27" diameter. Engine is 413 cu in (stroked 350) in a 1970 Firebird. Comp XE276HR cam, Torker I intake with 800cfm Q-Jet, Hedman headers, 2.5" Pypes X exhaust system, and TCI TH350 with a 2400 stall converter. Last time I weighed it with me and tank of fuel it was 3670 pounds. Dynoed at 442 LB FT@4300rpm

I'm extremely pleased with the drive train combo. The converter has just enough slip to make stoplight situations easy, yet is tight enough to provide 75 mph @ 3000 rpm. I was able to average almost 19mpg at a steady 70 mph on a 200 mile trip.

[tooski]

With my 4.11's and 27" tire at 65 mph I was running 3600 rpm. The engine seemed like it was buzzing a bit too much. It also seemed to get to 6k a little to easy. I'm looking to keep it around 3 or a bit under. I'll be going with the 3.23's.
Thanks for all the replies.