[ericwy]

What's a reasonable expectation for gas mileage for our 400 equipped TAs?
GM offered 2.41 gears for better mileage during post oil embargo 70's.
What kind of mileage to you all get with 2.41 or 2.73 gears with a 400 ?

I am getting about 15mpg city driving with the combo listed in my sig and I use
the secondaries ALOT. (sorry no highway mileage #'s yet) I was getting 11-12mpg
before putting in the OD 700R4. I am Planning on installing some 3.42 gears to bring
the final ratio up from 2.156 to 2.394 while in OD similar to GMs 2.41 final ratio.

[Philo]

I have the '79 with the 400 and 3.23 ratio. It gets 12 local driving. I have never checked it on a trip, and have only checked it locally once (last year).

[Don 79 TA]

i had a similar combo back in the day and was knocking 15-19 with T tops out
also had a small stall convertor too (ok, i'll admit it at one time i had an automatic lol)
i had a spreadbore holley on it
all things considering, i thought that was good
i wonder what it would have done with better tunning, and a 5spd overdrive
at that time i had the 2:73 posi

[Y81all]

I used to get 10 pretty regularly in a 77 with a W72-400/4spd combo with 3.23s.

[ve3wireless]

My stock '78 Trans Am --- with the T/A 6.6 engine, Borg-Warner T-10 transmission, and 3.42 rear end --- averages about 16 mpg (Canadian) in combined city & highway driving...

I swear, when you punch the accelerator hard at highway speeds, & all four barrels kick in, you can actually SEE the gas guage move closer to the "E" side of the scale! But I guess if all we were ever concerned about was fuel economy, then this forum would be dedicated to '78 Chevettes with four cylinder engines...right?!

Eddy

[95ramairformula]

Who really cares? I don't... If you have to ask you can't afford it. I have never checked mine.

[ericwy]

Originally Posted by 95ramairformula

Who really cares? I don't... If you have to ask you can't afford it. I have never checked mine.

I care about it. I have owned and driven this car since 1983 and I would like to continue
to do so. I make careful choices that provide a balance between performance and economy.
Its no race car. Its a driver, its a blast to drive and it turns heads.

Whether or not I can afford it is irrelevant. I am just asking for input so I can set reasonable achievable goals.
Just throwing money down the fuel tank because I can is not the point.
I have endless choices of gas guzzlers to choose from but I choose
a Disco Stu era fire doe-doe-bird because I think they are cool. Everyone knows there
is loads of performance potential that can be realized by choosing the right parts. There
is also fuel economy potential. All you have to do is look for it, which it what I am doing.

This thread is about getting optimal gearing to compliment the engine's power curve, but
I am also looking at ways to improve the fuel metering and dial it in so it works with my
combo.

Here's an article : http://www.chevyhiperformance.com/te...age/index.html

Thanks for the help.

[Skip Fix]

"I swear, when you punch the accelerator hard at highway speeds, & all four barrels kick in, you can actually SEE the gas guage move closer to the "E" side of the scale!" You should have seen what my old dual quad 409 did when all 8 barrels were cracked open

You know I don't remember what my 78 got when it had the 2.56s. But with the stock 400 it could only pull 4000 in high gear-going 130 mph.

[Don 79 TA]

Originally Posted by Skip Fix

"I swear, when you punch the accelerator hard at highway speeds, & all four barrels kick in, you can actually SEE the gas guage move closer to the "E" side of the scale!" You should have seen what my old dual quad 409 did when all 8 barrels were cracked open

You know I don't remember what my 78 got when it had the 2.56s. But with the stock 400 it could only pull 4000 in high gear-going 130 mph.

And all along i thought i that was the tach, E for excitment
dammit skip... now you took that fun out

although i love the way the tach and spedo swing in harmony

[74 SD455 Formula]

In 2000 when driving to the T/A Nat's from Wisconsin to Ohio I was getting 17 and a half miles to the gallon with a unrestored original 79 Formula with a 400 4spd and 323 gears.I had P295 50 15's on the rear and P275 50 15's on the front.

[Skip Fix]

130 was about the "P" in MPH on that low mph late model speedo.
I could run most of the road course at Texas World Speedway in second!

["QUICK-SILVER"]

ericwy,
The chevy article old school new school, is old school. Fine tuning carbs, has been around since carbs. A performance tune up, carb included, on an engine is for best power. Best power comes from burning fuel with the highest efficiency possible. Highest efficiency possible is going to give you the best gas milage.

Less load on engine is going to give you better milage. You found that out changing transmissions, engine doesn't labor as much taking off with lower gears.

When you start thinking about rearends for highway cruising, you will want to know your engines sweet spot, too much can be as bad as too little.

carb 101 another day
it's much easier than the chevy article.

[ericwy]

Originally Posted by QUICK-SILVER

ericwy,
The chevy article old school new school, is old school. Fine tuning carbs, has been around since carbs. A performance tune up, carb included, on an engine is for best power. Best power comes from burning fuel with the highest efficiency possible. Highest efficiency possible is going to give you the best gas milage.

Less load on engine is going to give you better milage. You found that out changing transmissions, engine doesn't labor as much taking off with lower gears.

When you start thinking about rearends for highway cruising, you will want to know your engines sweet spot, too much can be as bad as too little.

carb 101 another day
it's much easier than the chevy article.

Agreed on the historic nature of the chevy article, and I dont plan on modifying my
Holley PV channel restrictions and air bleeds etc... The two carbs I have to work with are a 4165 DP and the 4175 vac sec. Both are "emission legal" street versions with reverse idle mixture control. I am planning to check its current state of tune esp for cruise and part throttle.

As far as this thread goes its about getting the optimal gearing. How do you determine this? My engine peaks tq about 3400 but its a flat peak. I cant seen cruising at 3400 rpm is going to get very good mileage vs at 2000.

[bonneville Joe]

I never really tried but I know mine sucks lol.....

78 W72 4spd with 3:42s, 77 W72 auto Quadrajet and a Summit 2802 cam. I'll make a note of it and figure out what Im getting. Im gonna guess somewhere between 9 and 12 mpg depending on how I drive it.

[Peti972]

i get ****ty gas mileage to..it's hard not to be E-Z on the gas pedal know what im sayin lol..im guessing i get about 15mpg with a 2bbl carb

[ericwy]

FYI...
Just took TA for a 400 mile round trip. Average cruise speed about 80 mph
and averaged 17 mpg. Had to cruise high mph to hit a sweet rpm spot around
2100 rpm.

[bobzdar]

When mine was a DD, I'd get around 20mpg highway. 400/4spd with stock 6x-8 heads, 3.23 rear, 744 cam, torker 1, qjet and headers into 2.5" dual exhaust and lots of timing (38 total plus full vac advance). I left primary jetting stock (lean) and richened the secondaries to compensate under WOT. It worked well with the open plenum T1, when cruising around I'd be running really lean and get good mileage but when floored the secondaries would compensate.

I switched to #16 heads and an early iron intake with everything else the same and economy is down but it hasn't been tuned yet, so I attribute it to that, at least partially. The timing is down to around 32 degrees until I get the carb tuned (way too lean on primaries and no open plenum, so I know the mixture is uneven under WOT).

["QUICK-SILVER"]

Originally Posted by ericwy

FYI...
Just took TA for a 400 mile round trip. Average cruise speed about 80 mph
and averaged 17 mpg. Had to cruise high mph to hit a sweet rpm spot around
2100 rpm.

ericwy,
2100, thats the sweet spot I was refering to in earlier post.
You said about 80, pretty dang close, 78.36.
3.42 will drop you back to 70.57 at 2100. Thats not but 7.79 mph difference, will be mostly less wind resistance and probably not enough to see a big change in gas milage.

If you put the DP on and change to the 3.42 gear, you should have a major performance increase and with the smaller venturi on the DP (increased air speed better atomized fuel mixture) do some better on the mpg.
If you get the DP just right, 20 should be no problem.

[bonneville Joe]

Just driving around town at around 55 mph with my mentioned set up above I figured Im right at 12 mpg. I need to do some major tuning to the carb. It smells rich when Im sitting at a stop light.

[Peti972]

Originally Posted by bonneville Joe

Just driving around town at around 55 mph with my mentioned set up above I figured Im right at 12 mpg. I need to do some major tuning to the carb. It smells rich when Im sitting at a stop light.

is that what it is? lol i thought it was because the open hood scoop & hole in my firewall...