Fuel mileage question

[TMA62]

What is your average mileage that you are getting? My Avanti was just rebuilt and I think the carburator still needs adjustment. Please provide the following: Engine, Carburator and year.

I'll start off. I have a 1971 Avanti II with a GM 350 V-8 and a Rochester 4-barrel Quadrajet. I'm getting just over 10mpg (but I think it should be higher than that). My mileage is combined highway and city. If you want to break down your mileage, that woukd be great.

Thank you in advance!

[Gunslinger]

I would think your car should be doing better than that. I would expect 16-18 mpg even with a heavy foot. I've always felt carburetors are voodoo...I cannot understand their workings at all. I do know that Quadrajets are among the best carbs out there regardless of the nicknames given them..."Quadrajunk", Rottenchester", etc. Those nicknames come from cars from the middle and later 1970's when emissions laws were stringent and manufacturers had trouble meeting them with existing designs. It was easy to blame the carburetors. Q-jets of those years had limiter caps on the adjusting screws and other power and idle robbing attachments and jetting. That can apply to most all cars from that period. See what Mopar enthusiasts say about Carter Thermoquads and their warping plastic bases.

Assuming there's no vacuum leaks or other issues with the carburetor or induction system, you should take the car to a shop with an exhaust gas analyzer. That's the best way to properly tune the carb. Some techs...very few...can do a good job of tuning the carb with a vacuum gauge but the exhaust analyzer is the best way.

Other possible avenues to explore...wrong timing, distributor vacuum advance is wrong, vacuum leak, maybe some other cause but those would be the main things to check.

BTW - I have a '70 Avanti with a 350 330hp crate engine but I eliminated the carb and went with fuel injection. More expensive installation but eliminated a lot of carb related issues.

[PackardV8]

Which transmission? Which rear axle ratio?

[TMA62]

Thanks Gunslinger and Packard. I forgot to mention the trans. I have a Borg Warner AS2-12D which is a 3-speed but with no Overdrive. That obviously is effecting the mileage. I don't know the gear ratio off hand.

[Guest dapy]

Not sure my 2 cents is relevant. In the few months I have owned my 1989 I have dutifully checked the mileage. All is in-town, in extreme OAT (with A/C on), and in traffic. Best I can determine my car is stock: 305 w/carb; 700R4, but may have a lower geared differential. (Seems to have very good acceleration.) I just had a Dyno check (HP and torque) which appeared normal for the engine miles and age.

So what is answer? 14 to 15 MPG. Smooth running. Smooth shifting. With 'no push' automatic overdrive (4th gear) engages at about 30 MPH and reduces RPMs by about 600. I assume that is favorable to fuel consumption. I use premium gas. Should I?

Edited August 6, 2013 by dapy

[warren55]

There's a lot of opinions on which fuel grade to use, but I think that motor would be just fine on regular gas. It shouldn't need the higher octane, but there may be a few more additives in the premium gas. It would be cheaper to use a good additive once in a awhile, IMO.

[profaqualung]

My 70 when stock - with a 300/350 - 3.70 Dana rear - Stude. trans - would get 15.5 to 15.8 consistently - and I drive hard.

Now that it has a 383/425 [with extra headwork and different cam] - Ford 9" rear with 3.50 - and a beefed 700R4 - I am getting in the mid to upper 18 range at posted highway speeds - and 19.1 running 2600+/80+. Go figure.

The above stated figures are obviously highway. Around town it is in the 11 to 12 range. The 383 has a Barry Grant 850 carb.

When it was in its original configuration, I was getting miles per gallon in the range you are experiencing, but as stated, I drive hard.

Gunslingers suggestion on the FI is right on the money. Best of both worlds - performance and fuel economy. For an Avanti that will be driven a lot, it would be well worth the initial investment. If FI is not in your future, talk to the local car guys/hot rodders - find out who is the local Rochester guru and have him set the carb up, and stick with him getting it adjusted/tuned.

pa

An option you might want to consider, a GearVendor overdrive unit. Put one on my 70 Mercury Cyclone [boss466/720 horse w/C6 trans, 3.50 rear] and jumped from 15.1 on the highway to 18.6

[Guest dapy]

What's an FI? Would it apply to a 1989?

[Gunslinger]

FI...Fuel Injection.

[Guest dapy]

When did FI become standard? Dumb, but how do you know if you have it?

BTW what does Comp TA mean on the 'production order'?

[Gunslinger]

The fuel injection in my car is an aftermarket Edelbrock system. I'm not sure if late Avantis from the Caffaro era came with efi or were still using the computer controlled carburetor. Most likely they used whatever the Camaro was using, though Avanti Motors was often a year behind what GM was using.

To tell if you have fuel injection, see if you have a carburetor...if not, you have fuel injection.

I'm guessing "Comp TA" on the production order means BF Goodrich Competition T/A tires were specified for the car.

[Guest dapy]

I have a carb so it is likely a computer controlled carb. Works OK.

Yes, the 'Comp TA' must be tires. Long since replaced.

My 1989 was built on a 1989 Caprice Classic frame with the then standard 305 drive train. Works good.

[PackardV8]

FWIW, I enjoy playing with new toys as much as the next guy. However, if one does the math, we'd never live long enough or drive an Avanti enough miles to pay for an EFI or Gear Vendors overdrive from the savings from increased fuel mileage.

Yes, both make the car more pleasant to drive. No, they're never cost-effective. Maybe, define one's goals and budget.

jack vines

[Gunslinger]

I agree with Jack...converting to efi is not cost effective unless you pile up miles on your car. That issue wasn't important to me...I simply didn't want to deal with carburetors anymore. To me, the extra cost was worth it.

[plwindish]

My carbureted 76 rebuilt 400 SBC with a 200R4 gets 15 - 17 around town and 21.5 on the road driving @ 70 mph. 3:31 Twin Traction rear-end. New pistons were used in the rebuild along with a hydraulic roller cam, original heads were kept and cleaned up for better flow, new aluminum intake, 650 carb,MSD ignition and headers. Before the rebuild, 12.5 was it for mpg with the smog equipment and THM 400 trans.

[ernier]

Are you sure the speedometer and odometer are accurate. My son tried to convince me that his modified Lincon Mark VII was getting better mileage with the 3.73 gear he installed over the original higher gear ratio. His argument disintegrated when I suggested he check his odometer against a mile marker. Different height tires or a malfunctioning speedo can make your mileage seem much different than it actually is.

As far as the carb...make sure the choke is opening all the way, if the car has a heat riser make sure it's opening, if the carb was replaced it may not be properly jetted for your engine. Off the shelf rebuilts are not necessarily set up properly for the application even though they bolt on.

Your mileage does sound low especially if it's combined highway/city.

Any fuel smells after you shut it off? Fuel evaporates so quickly that only the biggest leaks show up on the floor.

Do you have the orginal air cleaner on the car and does it have a thermactor valve on the snorkel? If that valve doesn't open and you still have a hose going to the exhaust manifold you sre restricting air flow. If you have an aftermarket air cleaner is it a small diametr style. They are more restrictive than the 14" style.

Timing is critical to mileage. Vacuum advance needs to be working. Total advance at cruise RPM with the vacuum advance hooked up should be 45-50 degrees but can vary from engine to engine. Retarded timing hurts fuel mileage. Centrifugal advance needs to be working properly. Don't be afraid to experiment with initial timing as more advance may work better over the factory recommended setting.

Good Luck.

ErnieR

[drat]

In my 69 witj the 350 get about 11mpg with

[drat]

Whoops sent too soon.......

With the 350, quadjet, borg Warner 3 speed I get 11mpg average. Modifications are sanderson block hugger headers and 2.02 heads and HEI distributor. 70mph on the freeway takes the tac up 3000 I believe. Maybe 3500. Cant quite remember. Desperately want an overdrive.

[Charlie Kile]

On a recent 450 mile trip, my 84 Touring option Coupe got 24.9 MPG.

Engine; A stage 2 Jasper 350 with GM TPI. Apx 400HP

Tranny; 700R4 with a locking torque converter

Final drive; 2.86 gears with positraction.

At 70 MPH My tach shows just over 1700 RPM

Charlie

RQB3921

Edited November 20, 2013 by Charlie Kile

[PackardV8]

Whoops sent too soon.......

With the 350, quadjet, borg Warner 3 speed I get 11mpg average. Modifications are sanderson block hugger headers and 2.02 heads and HEI distributor. 70mph on the freeway takes the tac up 3000 I believe. Maybe 3500. Cant quite remember. Desperately want an overdrive.

You also desperately want a wideband O2 sensor and display. I tuned carbs for fifty years and only after I began using the wideband O2 did I learn how much I didn't know.

The 11 MPG is low even for your setup. The Q-Jet is capable of much more even at that RPM.

And yes, you do need to calibrate your speedometer and odometer. You have to choose one or the other to get most accurate. Seldom do both read correctly on a twenty-year old analog instrument.

jack vines

[drat]

Thanks for the advice!

[Avanti7]

I bought my first Avanti last june from a good friend in our Stude club. It's a 1970 350 bored .030 over with roller rockers and roller lifters. it's pushing 325 HP, with

a trbo 350 tranny and 3:31 rear gears. It also has 75 series tires. Drove it to Branson Mo. for an orphan car show a couple months after purchase, an 800 mile

round trip. With the A/C running in the afternoons the car averaged 15.5 for the entire trip, including driving around in Branson. I didn't think that was to bad for the

drive train I'm running. I was going to put a 700R4 in it but the engine and transmission were both rebuilt 8K miles ago, so I think I'll just sit tight.