Currently
1968 Firebird Convertible
Original 1968 350
Originally a 2 barrel car, replaced with Edelbrock 1406
Comp Cam XE262H-10
3 speed Turbo 350 auto
2.56 open rear end

Usage
I like the 22MPG that I currently get. 3,000RPM at 80 MPH. I do take 100+ mile road trips from time to time, but most of the driving is local and non-highway. I’d like more acceleration, but don’t want to be miserable on the highway. I’m thinking about replacing the drivetrain. I’m not too worried about keeping her 100% original, but would keep the 350 on a stand. I don’t mind the automatic trans, but could switch.

Question
What would be your dream drivetrain (motor, trans, rear-end/gears) given my desire for performance and highway-ability (is that a word? )? I want to keep it Pontiac powered. I’d like to keep it under $15,000. What would you choose and why?

196x/197x Pontiac 400/428/455?
Manual or automatic? Overdrive, etc.?
Posi?
Rear gears?

I would looooove to see some evidence of your 22 mpg's on a 68 Bird. Never heard of that. The documented mpg's from the factory were around 11mpg's avg in a 400, up to ~14mpg's on a 350. You claim you beat that by 50%?!?! Sorry, but I have to challenge that. If you have a secrete receipe it may be worth big$$$.

Was wondering that as well and stated in my reply to your other thread.

I'd be happy with 15 MPG. You must only drive downhill.

Wow, 22. I only get about 15 mpg in my Sprint.

The 2.56 rear can really help highway mileage. On it's last real long trip, my 400HO got 17.5 IL to CO and 19.5 CO to IL.

IMO, given your existing car and what you want to do, I recommend the following:

existing 350 if it does not require a rebuild
better Pontiac heads
higher performance cam 068??
standard transmission /w overdrive (new tech transmission)
auto trans /w overdrive and get shift kit installed so you can shift rears with automatic
3.x rear gears
4 bbl Q-Jet or new tech fuel injection
stock intake (for Q-Jet)

It is entirely possible to get 22 mpg with EFI and overdrive trams, but the expense would be a wash unless you drive the car everyday till you die.

So, on the documented mpg’s from the factory, what gears were they using? Personally, I’ve never seen factory documentation on mpg per every gear option. I’m running at approximately 2,500RPM at 70MPH. I don’t know what to say about 22. It’s pretty simple math. However, if you’re up for a road trip for proof, come on up to Michigan.

Just to be clear, maintaining 22MPG isn’t my goal. I’d like more performance,but not at the expense of being able to do 70MPH on the highway somewhat comfortavly. Perhaps, should have asked what would be your dream combination of rear end gears and transmission.

I can fry my tires as long as I want and cruise at 70mph at about 2200 rpm. 3:55 posi rear and 700r 4 tranny. But sadly, I don't get 22mpg. ( nor does any other member of this forum l'd bet)

Might be easy math, but I know BS when I read it.

As Gus68 said above a basic upgrade package on the 350 will get you some pop. Intake, carb, duel exhaust or headers, maybe some new heads if you get serious. Swap the gear to 3:08 posi. You'll still cruise just fine.

Sorry, but true. 1,500 mile highway road trip. 22MPG. No point in continuing to defend the MPG. I’ve put almost 8,000 miles on it this summer. I know how to calculate it. It really wasn’t my question anyway. For those of you that actually read my post, could understand the question I was asking and answered in a constructive way, thanks for your help. Go Pontiac!!!

I am putting my 69 Ram AIr IV engine in my numbers matching 68 350 HO 4 Speed Coupe. Ram Air IV, 4sp and factory Safety-T-Track rearend equals fun all day long and uping the tire budget.

0:50


https://www.youtube.com/watch?v=4bq73voK5dA

TLC , you've got to be able to understand the reaction to such a claim. Members here have been driving their Firebirds for...decades?...with possibly an identical drivetrain to yours and not achieve anywhere close to that mpg. Great for you if you do. Don't be offended if few believe you. I trust all can read and comprehend your question.

For $15k, you should easily get about 500HP from a stroked 400, maybe with aluminum heads...Add a built 700r4...and a 3:55 rear end. However, your fuel and tire expense will go up.

...or just the tranny an rearend. The 700r4 will have a lower 1st gear, along with the 3:55 rear. You'll be quick on the bottom and quiet
At 70+ mph. ...and save a ton of cash.

I'm running a stock 350 with a fresh rebuild, quadrajet and 5 speed with .63 overdrive and 3.08 gears. Rpm's@ 70 mph are about 2000. Best I've ever been able to do between gas stops on the inter-state is just under 20 mpg. That is traveling at 75 mph. Maybe if I kept it at 55 I could improve on that but why. Around town it is way less.

Awesome!