[ericwy]

I know mileage considerations usually take a back seat with our cars, but I have been tuning for gas mileage and performance and got some good results:

The stuff: (see sig for more)
TA with 400 engine has 0.7 overdrive + lockup converter. Final ratio is 2.39:1 so engine rpm is about 2K at 70 mph.
Holley #7054 4165 body 650 cfm spreadbore double pumper. #59 jets primary jets + 2 stage PV (10.5"&2.5"), #582 secondary jets + 6.5"PV
Engine timing : 16 dgrees BTDC + 20 degrees mechanical advance by 2800rpm, 12 degrees(mechanically limited) vacuum advance @10" manifold vacuum
Fuel : 93 octane pump gas.
Idle - 15:1 A/F, 15" vacuum at 700 rpm
Cruise - 15.7:1 (>10.5" vacuum) 5mph to 45 mph
Cruise - 15.5:1 (>10.5" vacuum) 45mph to 80 mph (maybe squeak out another 1/2 percent getting it to 15.7:1 A/F by dropping to #58 jets)
Part throttle - 14.5 to 14.8 (<10.5" vacuum) on 1st stage of 2-stage power valve
Part throttle - 12.5 (<3" vacuum) on stage 2
WOT - 12:1 (or any time the secondaries are opened, PVCRs are bit too big)

The test:
Fill ups are at same station, same pump, same time of day (morning) each time. Fill until pump stops.
City driving is to-from work following same route and time of day (time selected to avoid stop-n-go traffic for best case)
hi-way driving is 80 mile round trip loop following same route and time of day.
Driving style for best case is "Grandma" keeping engine vacuum > 10" as much as possible, light accelerations and coasting to stops.
NO AC

Best city mileage : 15.27 mpg (averaging 40 mph over 20 minute commutes, no AC)
Worst city mileage : 12.24 mpg (stop-n-go traffic with AC, or with aggressive/spirited driving)
Hi-way mileage : 21.56 mpg (averaging 65 to 70 mph, no AC)

While it is possible to see some improvements in city mileage I think its hugely dependent on traffic conditions and driving style.
Most of the benefits of my carb tuning efforts improved throttle response and performance. Response is much crisper and I use less throttle most of the time.

YMMV

[Scarebird]

This and getting high 13's? not bad...

[Silver Judge]

Good job on your choice of parts and your tuning here !

Like Scarebird says, all this and you run 13 something in the quarter mile, that's damned good I say as well.

On the streets and highways, it takes some serious will power to stay out of the secondaries. Ask me I know

[67drake]

I think this shows what a well thought out combo can do. Congrats.

[Half-Inch Stud]

Congrats indeed. A orthy goal for some of us doing the OD upgrade.

[b-man]

Nice going, I'd bet a well-tuned Q-jet would bump it up by 1 MPG or so.

[ericwy]

Thanks, its been a lot of trial and error (mostly error) before finally sitting down and doing the research. I tried for years using a 3.08 axle behind the OD tranny and running super lean mixtures. Ended up with a car that cruised below its torque range, was slower, ran hot, had detonation problems, and had smelly exhaust. I finally started to listen to you guys and went back to basics.

[Cliff R]

'Nice going, I'd bet a well-tuned Q-jet would bump it up by 1 MPG or so.'

I've had customers report 17-20mpg's from 455's installed in later T/A's with the mid 2's gear ratios and no overdrive.

One very competent tuner maxed out his Holley carb using an 02 meter, and did some back to back testing against a Q-jet we set up for him. Right at 2mpg's better with the q-jet. He ran the same course, filled up and the same station, etc......Cliff

[69Customs]

What size tires you running?

[Captainofiron]

Not bad, I hope mine is that good

[rohrt]

Very ecouraging results, thanks for posting it up.

[455Grandville]

Originally Posted by Cliff R

'Nice going, I'd bet a well-tuned Q-jet would bump it up by 1 MPG or so.'

I've had customers report 17-20mpg's from 455's installed in later T/A's with the mid 2's gear ratios and no overdrive.

One very competent tuner maxed out his Holley carb using an 02 meter, and did some back to back testing against a Q-jet we set up for him. Right at 2mpg's better with the q-jet. He ran the same course, filled up and the same station, etc......Cliff

My 75 GV with a warmed over 455 has done 19 with 2.56s , and it's pretty hilly where I drive it -

eric: what kind of $$$ did that overdrive cost ?

[Erik_H]

That's impressive, and encouraging. I've been slowly improving my 350 stroker and this proves I still have a good bit of room for improvement. You're about 4mpg better in all cases. If you think about it, you're better than at least 1/2 of the newer cars/trucks on the road today.

I am very interested to know more about what equipment and how you've used the A/F meter to aid your tuning. I think that might be my next learning experience.

[TheSilverBuick]

Nice!

You're making more HP than I'm sure my old '69 Firebird was but I had great mpg's with my old '69 Firebird. It had a very docile 400 (compression/cam unknown), tuned up and adjusted the Q-jet, points distributor, installed a 700r4 with lockup. 2.73 rear gears. Regular 87 octane fuel it would get 18mpg around town and 25+mpg highway. This was over a period of two years of daily driving it to college and around. It was great for college! Looked good and got great mileage. It pulled 17inHg at a 700rpm free idle, 15inHg 600rpm in gear idle, some times more if the weather was right. Cruised down the highway pulling around 12-14inHg.

[rasilverbird]

Very nice.

FWIW I got 16mpg highway once (normally 15) in my GTO when it was restored to stock. 400 with TH400 trans. Drafting other cars really helps

[ericwy]

Originally Posted by 69Customs

What size tires you running?

P255 60 R15 (27" diameter) Firestone Indy Firehawks

Originally Posted by 455Grandville

My 75 GV with a warmed over 455 has done 19 with 2.56s , and it's pretty hilly where I drive it -

eric: what kind of $$$ did that overdrive cost ?

$1600 for the tranny and lockup converter in 2007. The Th350 stopped working so upgraded to the 700. To be honest though,
I was more interested in the deeper 1st gear for better acceleration at the time!

Originally Posted by Erik_H

That's impressive, and encouraging. I've been slowly improving my 350 stroker and this proves I still have a good bit of room for improvement. You're about 4mpg better in all cases. If you think about it, you're better than at least 1/2 of the newer cars/trucks on the road today.

I am very interested to know more about what equipment and how you've used the A/F meter to aid your tuning. I think that might be my next learning experience.

AEM Analog Wideband Air - Fuel Gauge kit for $153 at Autoplicity.com

Its a great tuning tool. It replaced an old narrow band O2 gauge which I learned is useless for tuning. The wideband showed the it was dumping excess fuel through the carb at all times. The AEM even revealed during braking fuel was sloshing up through the secondary vent tube due to a high float setting. The AEM gauge reaction time is lighting fast, it easily registers the accelerator pump shot when blipping the throttle and any other transient changes.

Originally Posted by TheSilverBuick

Nice!
...Regular 87 octane fuel it would get 18mpg around town and 25+mpg highway.
...

Wow! that's awesome.

Originally Posted by rasilverbird

Very nice.

FWIW I got 16mpg highway once (normally 15) in my GTO when it was restored to stock. 400 with TH400 trans. Drafting other cars really helps

That's about what I used to get with the th350 + 3.08 and an untuned Qjet.


Funny thing about doing this was it improved seat of the pants performance which makes it more enjoyable to drive. I wasn't even thinking about that and assumed improving mpg would make it run crappy. Completely the opposite. Before tuning my car was only working well at WOT, and even then it was too rich (still is).

I have some more tuning to do for WOT (leaner) and the primary pump shot needs refinement (it shoots ALOT of fuel every time the throttle moves)

[geeteeohguy]

I consistently get a bit over 20mpg@ 75 mph with the 400 in my '67 with a TH400 and a 2.56 rear gear. No overdrive. It has 87cc small valve heads (#15's) and stock exhaust. It sure won't touch 13 seconds in the quarter, though!!!