I'm only getting 12 mpgs at best with my stock 400/400TH...

http://www.oneownercollectorcar.com...ontiac-firebird-400-ho-4-bbl-mpg-13-city

The mathematical equation for high mpg is high compression + high torque + low rpm = higher mpg. That's why diesels and hybrids do so well. Todays 4 cylinders are race engines to get high mpg, they are running high compression again which you can with poor fuel and a computer.

The problem with older cars is rpm, I'm running a RAI 400 with 3:42 rear and a stock 4 sp, I get 8-12, I do 1000 miles a year and I am generally cruising at 2000rpm, I don't drive it enough to care, but if you got a 5 sp or an electronic over drive kit and went easy you'd be @20

TH 400s suck away a lot of power. A manual trans or even a TH 350 would help.

It's not worth the time or money, how much do you really drive the car??? Like I said I do a 1000 a year,

If you went from 12mpg to 16mpg you would go from using 83 gallons to 62 and from spending $250 to $186, it would take 300 years for it to pay you back.

Drive the car and enjoy it.

Yes, the point of these cars is to burn massive amounts of 93 octane gas for short periods while throwing your passenger back into their seat.

I can make 2 points on both sides of the mileage story - both being "modern" vehicles.

1994 Chevy 3/4 ton extended cab 4x4 pickup; 454 ci TBI; 240 hp; 4-speed AOD tranny; 4.11 rear axle; 10,000 lb towing capacity. 10.5 mpg no matter what. But I commuted to work in this vehicle for many years for the following reasons:

1) It sat outside in Tornado Alley. Due to it's age, if it ever got hit by a hail storm (the biggest weather threat) I would get practically nothing for it from the insurance company. Because I bought it with very low mileage, I figured I better get the miles out of it.
2) Gas prices are traditionally pretty low in Oklahoma.
3) I carpooled. Thus I drove only 1 or 2 weeks a month.
4) I got even cheaper gas due to having a grocery store discount card. I believe it's 10 cents a gallon off for $100 of groceries.

Replacement for the truck above: 2019 BMW X3 AWD; 2.0 L Turbo 4 Cylinder; 250 hp; 8 speed auto tranny; Premium gas only; 4100 lbs weight; 4400 lb factory towing capacity. Accelerates better than my lighter, computer tweaked V6 Mustang and is knocking down 28 mpg so far. An example of modern vehicle engineering.

Aye, I remember the fascination of punching some of my old cars to watch the gas gauge go down.

I had a 68 Cutlass with a built 396, I forget the heads but it had the 510/320 cam like the RA II and IV, it got 5mpg, the car was unreal, someone else had built it and it was one of hose local cars with a reputation that as a kid you bought because of that, if I hadn't spun the #8 rod bearing I probably would have killed myself in it

I got one even better...2015 VW Golf Sport wagon TDI. I bought it the night before the news hit that VW was cheating the smog with their software. It's a "Gen 3" so we were able to have the fix done. We drive it from northern cal to Ventura in So Cal about every couple of months. It's a 2.0L Turbo with 6sp auto. We usually avg 50mpg. That thing rolls at 90mph without even breathing hard and will fry the tires from a stoplight.

Awesome

Don't forget the lead foot factor in MPG. It takes very little to keep ole Betsy moving down the road. That big engine purrs right along. But getting her up to speed in a different matter especially if you like to do it in a hurry! This difference is gas mileage is really quite remarkable. But who cares!!! I heard once that is takes less than 20hp to keep a ferrari going down the track at 100mph... once it reaches 100mph. That might be an old wives tale but I think the point is made just the same!

also pending your rear axle gear...mine was 2.56 rear...
I got 14 MPG cruising in my 400. (at 70-80)..9 around town...

When I switched from 4 spd. to 5, the overall mileage didn't change enough to be noticeable.
I can get close to 20mpg on the highway. It is a 350 re-built to original specs (2bbl) with a quadrajet, 3.08 rear and .63 overdrive. Around town it goes way down.
I didn't do the transmission swap to save money, I do a lot of highway travel and don't like cruising at 3500 rpm.

That is interesting reading, a good job of documentation as well.

I think the mileage interest in that is Firebird specific.

I have a 51 Harley Panhead, a 68 Firebird and a 86 Ford F250 4X4. None gets good gas Mileage. I could sell it all and buy a Euro or Oriental car that gets very good mileage, or even an electric car. But I'd rather get poked in the eye with a stick.That's just me.

My GTO 400RA got six MPG, it was built to race.

When I build my 428 engine I had no choice on the bore as it needed to be bored 65 over to clean up the cylinders. I did have a choice with the shape and construction of the pistons, the stroke of the crank, type of lifters and rockers but most important: the camshaft. I could have built for high revving high horsepower engine but wanted more torque. I settled for a mere 535 HP but got almost 600 pounds of torque. Mostly due to the camshaft choice. So have 10.49:1 compression 463 cubic inch engine in a 68 Firebird coupe with a five speed manual, 373 rear 28 inch diameter rear tire and I get 26 miles to the gallon on the highway going way above the speed limit with the trunk completely full and two people on board. Could probably get a lot better at the speed limit but why would I have a 68 Firebird if I was going to travel at the speed limit. May as well take a bus.

I had a 72 GTO with a 400 4sp and I think a 3.55 rear and I got pretty good mpg as I remember, it was in the teens and @ 20 on the hiway, not sure why.