Enormous fuel consumption V-Code Fresh full engine re-build

[ZaraCuda]

I have owned my 'Cuda since 1989. It never got better than 11 MPG. I had the 4:10 Dana RE changed out for a 3:54. Not much difference. Fuel economy over the years deteriorated to around 5 MPG.
Engine full rebuilt to stock specs. Driven about 500 miles. No high performance parts used. Fuel consumption is miserable now. Maybe 3 MPG! Engine run on a dyno and properly tuned. Can't see any leaks. What is the most likely cause and cure? Thank you. Dave

[jimynick]

I'd park it for a while, and use the saved fuel money to purchase a wide band O2 sensor and bungs. Weld a bung in each side exhaust as close to the end of the manifold or header collector as possible and set up the sensor. They can give real time readings and then you'll know for sure what the hell is going on. There's no way the thing is tuned properly and gives 3mpg. check vacumn readings and compression and leak down to eliminate cylinder sealing issues. Look down, with a good light, the carb when it's idling, is it piddling anywhere? Look at the plugs, are they blacker than a foot up the chimney? Look at the tailpipes, are they also sooty? I wouldn't've thought it would use a gallon of gas if you put the pedal to the metal for those 3 miles, let alone just driving it. Just some pensees for you to ponder. Good luck 
PS- welcome to the site from Ontario, 

[70chall440]

Fuel economy is about efficiency, one side is the engine and drive train and the other is the fuel itself.

"Built to stock specs" doesn't really impact the fuel economy side; back in the day they didn't get great mileage either but gas was cheap and much better than what we have today. I would argue that "stock specs" is probably worse for mileage than a more efficient performance build.

Poor mileage is a direct result of the carb, the ignition and the exhaust as well as the quality of the fuel. Using all stock parts is going to make getting good mileage difficult.

I agree that getting an O2 sensor and tuning the engine with that is a step in the right direction. If you want better mileage you need to improve the ignition system and perhaps the exhaust system. You can also try changing the heat range on the plugs, tweaking the timing a little as well.

You also need to rethink those 3.54 gears if you really want "decent" mileage; might think about something like 3.23's. Lastly, you need to ensure your tires are fully (or even over) inflated to reduce the surface contact patch a little.

Good mileage and classic muscle cars are not synonymous typically although it can happen. If this is an issue for you and you really want to drive the car a lot, think about going to EFI and a good ignition like MSD.

[ZaraCuda]

Thank you for the solid advice. I live in the San Diego and Orange County, California area. Can anybody suggest a V-code expert shop that can help me get to the bottom of the problem?
I appreciate your welcoming me.
Sincerely appreciated!
Dave
 

[70chall440]

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