[3tas4me]

I realize that many of us don't care if we get poor fuel economy as most of our cars are not daily drivers anyway. That being said, I just about have finished up with the restoration on my 1974 T/A. The car is a 400 4-speed with a completely stock engine that has only 50k miles. The only mod to the car is the PYPES X system with the true dual crossover muffler. I have started driving the car and so far am only getting 12.5 mpg with highway driving.

I know a 400 will do better than that. 15-17 is what I expect for highway driving, especially with the more efficient exhaust. What are the rest of you getting with your 400 4-speed cars? My 1971 455 HO gets the same mileage as this. I know this car should be doing better. RPM at 70mph is about 2750 with the P255/60 tires I put on the car. My 71 is turning over 3000 RPM at that same speed yet gets the same mileage.

[padgett]

For best MPG want to be turning 1900-2100 rpm at 70 mph. Also need to get the proper instrumentation (tail pipe sniffer) and measure the AFR, at cruise I look for 14.7-15.7:1. 1974 was the last year before catalysts and engines were tuned to misfire. What is you timing (vacuum + mechanical) at 2500 rpm ? Also what rods and jets are in your QJ ? Been a long time but think 70 jets are pretty good all around. Secondary spring and butterflies need to be tight enough that it does not open at all at a 70 mph cruise.

Good MPG takes a bit of work and instrumentation. Recall that all around 1973 was the worst year for average MPG and '74 was not much different.

[tom s]

2X padgett,you have to get you final highway gear RPMs around 1900-2000.Start there and fine tune afterwards.Tom

[3tas4me]

Originally Posted by padgett

For best MPG want to be turning 1900-2100 rpm at 70 mph. Also need to get the proper instrumentation (tail pipe sniffer) and measure the AFR, at cruise I look for 14.7-15.7:1. 1974 was the last year before catalysts and engines were tuned to misfire. What is you timing (vacuum + mechanical) at 2500 rpm ? Also what rods and jets are in your QJ ? Been a long time but think 70 jets are pretty good all around. Secondary spring and butterflies need to be tight enough that it does not open at all at a 70 mph cruise.

Good MPG takes a bit of work and instrumentation. Recall that all around 1973 was the worst year for average MPG and '74 was not much different.

The only way I will get the RPM down that much is to put an overdrive Trans in or taller rear gears, neither of which I really want to do. What do you think I should be getting as is? Factory 3.42 rear. Again, it should do better than a 455 HO that turns more RPM and it isn’t.

I am currently reviewing my vacuum hose routing. Seems to me that car may not be getting proper vacuum advance as it feels as if something is holding it back a bit. Maybe a bad vacuum advance solenoid? I have a copy of the vacuum hose routing and it looks like both a ported and manifold source go to that solenoid. That doesn’t seem right to me.

Q-jet is stock with factory jets. I will have to check my timing as I don't know what it is at 2500.

["QUICK-SILVER"]

This car should be turning 3,000 RPM @ 70 mph. Your 2,750 RPM should be just a hair under 65 mph.

Odometer/speedometer reading fast makes mpg 'math' come out lower than it really is.

"something is holding it back a bit"

Need to check everything under the hood that can hurt gas milage.

Timing and vacuum advance. That vac advance does use two sources. Needs to be working right or plumb it so it does work right. Those engines also like a little more initial timing.

Make sure the choke is opening all the way.

Make sure the heat riser valve in the exhaust is opening.

Make sure the thermal vac door in the breather snout is opening.

If it's got duct work for cold air.. Check for obstructions. I've found dead birds in the plastic piece behind the battery.

Run more air pressure in the tires if you can. I go by what the tire says for max pressure to get better gas milage. 60 series tires are hard on milage and worse with low air pressure.

If you're having to crack the 4bbl open to cruise at 2750 rpm.. Expect everything above to not be working right

Clay

[FrankieT/A]

5 speed...

[tom s]

Gas mileage was not important back then,performance was.Without getting RPMs down the little stuff will help but not as much as RPMs.Yes OD is best.My 62 GP 421 4 speed car has 2.68 in the back but has a 3.42 first gear.Best of both worlds.Tom

[FrankieT/A]

If car manufacturers used OD in the mid to late seventies maybe they would not have had to smog their motors down so much. Its not rocket science. Look as us today automatics with 8 speeds, now they've gone overboard!!!

[3tas4me]

Originally Posted by "QUICK-SILVER"

This car should be turning 3,000 RPM @ 70 mph. Your 2,750 RPM should be just a hair under 65 mph.

Odometer/speedometer reading fast makes mpg 'math' come out lower than it really is.

I have checked my speedometer against the speedometer app on my phone and it is about dead on.

"something is holding it back a bit"

Need to check everything under the hood that can hurt gas milage.

Timing and vacuum advance. That vac advance does use two sources. Needs to be working right or plumb it so it does work right. Those engines also like a little more initial timing.

I believe my distributor vacuum solenoid is not functioning properly. I just tested it and it doesn't seem to work. It seems it is stuck in the bleed position so that means the vacuum advance never sees any vacuum at cruise.

Make sure the choke is opening all the way.

Working fine.

Make sure the heat riser valve in the exhaust is opening.

This car doesn't have that valve.

Make sure the thermal vac door in the breather snout is opening.

Working.

If it's got duct work for cold air.. Check for obstructions. I've found dead birds in the plastic piece behind the battery.

Duct work is completely clear.

Run more air pressure in the tires if you can. I go by what the tire says for max pressure to get better gas milage. 60 series tires are hard on milage and worse with low air pressure.

I typically run 34-35 PSI. That is where they are now.

If you're having to crack the 4bbl open to cruise at 2750 rpm.. Expect everything above to not be working right

Clay

See above for notes.

[3tas4me]

Originally Posted by FrankieT/A

5 speed...

Yes, I know.

However, this is not where I am going with this. Car is getting poor mileage set up as it came from the factory. After doing some testing, this afternoon, it appears the distributor vacuum solenoid is not functioning and instead is stuck in the bleed off position so that the distributor is not getting vacuum. I have bypassed the solenoid but will need to test drive to confirm.

[Aus78Formula]

Meanwhile, all the EGR equipment that people tore off as power robbing, only functioned during warm up like a choke. Nothing to do with engine power, rpm, gear ratio, colour...

[padgett]

BTW I ordered my '72 Goat wagon specifically with 400-4bbl (no fiber gears) THM-400, and a 3.07 "economy" posi. In winter ran H-78x15 snow tires.

Form memory got 18 mpg with no tow or a/c. 15 mpg with either tow or a/c on. and 12.5 mpg with both. Was turning 3,000 rpm at 70 which I always felt was too high but was best I could do back then.

Then the NMSL came in and wasn't cruising at 70ish any longer.

[FrankieT/A]

If you're at 3000 RPM you can tune til you're blue in the face...It will only get so good, and you will never reach what it was getting when brand new all parts being constant. At 3K RPM you will have increased fuel consumption and creeping temperatures not to mention way more noise. And you if have A/C, another story all together. Now, don't get me wrong your car should be in the best possible tune at all times, but facts are facts. If the car is all original maybe mothball the 4 speed and slip in a 5 speed making sure you don't do anything irreversible. There are a lot of things I did to my non matching numbers car that I can say was one of the best things I ever did(like hydroboost, 12 inch rotors, etc.), but the 5 speed was without a doubt the best thing I ever did. Now, I cruise at 80 at about 2300 RPM, my gas mileage is at about 18-19 on the highway, my engine temp stays around 185* and my A/C is ice cold.

[3tas4me]

Originally Posted by FrankieT/A

If you're at 3000 RPM you can tune til you're blue in the face...It will only get so good, and you will never reach what it was getting when brand new all parts being constant. At 3K RPM you will have increased fuel consumption and creeping temperatures not to mention way more noise. And you if have A/C, another story all together. Now, don't get me wrong your car should be in the best possible tune at all times, but facts are facts. If the car is all original maybe mothball the 4 speed and slip in a 5 speed making sure you don't do anything irreversible. There are a lot of things I did to my non matching numbers car that I can say was one of the best things I ever did(like hydroboost, 12 inch rotors, etc.), but the 5 speed was without a doubt the best thing I ever did. Now, I cruise at 80 at about 2300 RPM, my gas mileage is at about 18-19 on the highway, my engine temp stays around 185* and my A/C is ice cold.

My car can idle all day at 90 degrees and never get warm. As I posted earlier, I am turning about 2750 at 70 mph with my gears and 255/60 tires. No A/C on this one. I will see what happens now with functional vacuum advance. I bet that makes the difference. I am good with 15 MPG on the hwy, 12.5 is 455 territory.

[U47]

Originally Posted by FrankieT/A

If car manufacturers used OD in the mid to late seventies maybe they would not have had to smog their motors down so much. Its not rocket science. Look as us today automatics with 8 speeds, now they've gone overboard!!!

Actually the very hard years for EPA emission certification was the years 1972-74, really critical was 73-74. In those years we had high standards that had to be done ALL on the engine. In 1975 we all breathed a sigh of relief because the Catalytic converter allowed us to fatten up our CO & HC for better drivability and the Cat took care of the rest, instead of it all trying to get the job done on engine controls. The 73-74 were a nightmare trying to pass EPA. A lot of late nights on the dyno and in the lab.

This statement by Padgett is untrue;
1974 was the last year before catalysts and engines were tuned to misfire.

We could never get a certification if we tuned the engines for a misfire. Once you get a misfire or what he's talking about is a lean misfire the HC's go off the chart. A FAIL.

[LPete]

Originally Posted by Aus78Formula

Meanwhile, all the EGR equipment that people tore off as power robbing, only functioned during warm up like a choke. Nothing to do with engine power, rpm, gear ratio, colour...

EGR was turned off with the engine cold, not on. Driveability suffered with EGR activating before the engine was warm.

[U47]

Originally Posted by LPete

EGR was turned off with the engine cold, not on. Driveability suffered with EGR activating before the engine was warm.

When EGR was introduced (1973-74) depending on models, it was used on warmed up engines to lower the combustion temps to stop detonation on part to mid throttle. Lowering the combustion temps also lowered NOX and allowed us to run more advanced ignition timing. The TVS controls it's operation from cold to warm up, since the vacuum signal comes from ported vacuum even when the TVS is open there is no vacuum signal at Idle. At wide open throttle there is no vacuum signal so EGR is inop. in this mode also.

The addition of Catalyst allowed us to tune the engines to pre 1972 standards for better drivability. They would typically be 1- 1/2% CO and 200PPM's HC at idle before the Catalytic converter, and downstream of the Cat the Converter would reduce it out the tailpipe to as low as 1-10th % CO and 20-60PPM's HC. A huge reduction of emissions. That's why is against Federal law to remove Cats or any emission devise on 1975 or newer cars. Another thing Cats do is save exhaust systems. Usually a exhaust system wears out or rust out the tailpipe first, then the exhaust pipe ( a 2 to 1 ratio ) catalyst heats up the tailpipe to such extent that even short trip driver burn off condensation with no problem. My 1976 Olds which I bought new still has it's complete original exhaust and has no problems meeting the state test ay 116,000K.
In California it's illegal to remove any devise on any car with exhaust emission devises. Exhaust emission devises started in 1966. The reason for this is if the ARB or the AQMD can't meet their self imposed goals emission wise or their carbon offset goal numbers that these years of cars (1966-1975) can be brought back into smog check anytime WITHOUT referendum.

[78w72]

Originally Posted by 3tas4me

My car can idle all day at 90 degrees and never get warm. As I posted earlier, I am turning about 2750 at 70 mph with my gears and 255/60 tires. No A/C on this one. I will see what happens now with functional vacuum advance. I bet that makes the difference. I am good with 15 MPG on the hwy, 12.5 is 455 territory.

this is obvious but... lower the speed to improve MPG. when these cars were new the average highway speed was 55mph, when doing 70mph MPG gores out the window. so at those speeds & 2750rpm, 12.5 is not really too bad.

i have a 78 W72 4 speed with 3.42s & its been awhile since i calculated MPG, but at 60-65mph with 28" tall rear tires im doing about 2500rpm & get at best 14-15mpg, the TVS & EGR are deleted & i run ported vac to the vac advance, the engine isnt stock though, mild XE268 cam, .030 over, cliff built Q-jet & headers with 2.5" full exhaust.

when i do hit the interstate or even highways ~60mph is about as fast as i feel comfortable for prolonged trips, 70mph is just way too high of rpm for any real distance. i know the car/engine is fine at that rpm but almost 3000 is way higher than these cars were intended to go back then to get any decent MPG.

reading other threads about MPG in these cars with that gear, you arent too far off, many people report 13.5-15 mpg at those speeds/rpms.

[3tas4me]

Originally Posted by 78w72

this is obvious but... lower the speed to improve MPG. when these cars were new the average highway speed was 55mph, when doing 70mph MPG gores out the window. so at those speeds & 2750rpm, 12.5 is not really too bad.

i have a 78 W72 4 speed with 3.42s & its been awhile since i calculated MPG, but at 60-65mph with 28" tall rear tires im doing about 2500rpm & get at best 14-15mpg, the TVS & EGR are deleted & i run ported vac to the vac advance, the engine isnt stock though, mild XE268 cam, .030 over, cliff built Q-jet & headers with 2.5" full exhaust.

when i do hit the interstate or even highways ~60mph is about as fast as i feel comfortable for prolonged trips, 70mph is just way too high of rpm for any real distance. i know the car/engine is fine at that rpm but almost 3000 is way higher than these cars were intended to go back then to get any decent MPG.

reading other threads about MPG in these cars with that gear, you arent too far off, many people report 13.5-15 mpg at those speeds/rpms.

I should clear up the fact that my 12.5 mpg is both at 55 and 65-70 mph. Speed doesn't seem to matter. My 1979 W72 T/A has the factory 3.23 gears and a 4-speed with the only mod being a true dual exhaust system. I am able to get 17 mpg with that one. 15-16 mpg shouldn't be out of the question with the '74.

Yes, these cars should have been built with overdrive. That would have been an easy solution to increase gas mileage. Maybe someone can chime in why GM (and others) didn't pursue this solution?

[Sirrotica]

It wasn't until the feds implemented CAFE (Corporate Average Fuel Economy) penalties for car makers that they started to work on raising fuel economy standards.

The pellet style catalytic converter that came out in 1975 was just a bottleneck for the exhaust too. Removing the converter (test tube) gained about 1-2 MPG without any other modifications. The push for fuel economy did away with the pellet type converters, and companies went to the honeycomb style cats, lockup torque converters, fuel injection and the CCC, Computer Command Controls, with an ECM. That's when the fuel averages went up dramatically.

The larger V8 engines were also downsized too. The 301 debuted in 1977 for Pontiac, later followed in 1980 with a 265 engine. All the GM divisions downsized cubic inches after 1977 in pursuit of CAFE.

In the 80s the feds targeted trucks too and GM started using the OD transmissions in their trucks too, along with offering diesel engines in the light trucks to further drop CAFE averages. They tried diesels in the cars too beginning in 1978, but the less than stellar reputation of the 5.7 diesel in GM cars ended in 1985.

BTW, I owned an 81 Bonneville diesel, and taken care of properly (I then worked at a GM dealer as a line tech) they got really great fuel mileage. On a trip from Erie PA to Phoenix AZ the car averaged just under 32 MPG with the diesel running at peak efficiency. It sure wasn't fast at 125 HP (my 3.1 V6 GP has more power, 170 HP), but the fuel mileage of the diesel Bonneville and the 99 GP are pretty close on the same trip, the diesel got about 2 MPG more.

FWIW, I drove a 68 Bonneville with a 400 4bbl/T 400 in it, and with the highway gears, 2.56, and higher compression, advertised at 10.25 to 1. The Bonneville got about mid 17 MPG on the open road.

The lower axle gear, higher RPM at cruise, is what's hurting your mileage. A OD transmission, or lower numeric rear axle would probably help a lot, along with low restriction exhaust, and reworking the timing curve, and jetting. Anything you can do to keep air from dragging on the chassis helps too, front air dams, etc.

Every little thing helps out as the car companies figured out when the feds enacted the CAFE average.