Thanks DI, it would be nice to change that typo some how.....
This is a good start but not all of the story. You have to be willing to make a few more changes to tune the engine and make it function better.
To answer your question directly, my average local driving mileage was 18 mpg. After the rebuild it went to an average of 22+ mpg.
I have a very heavy car 3380 lbs with a full tank, 3600 lbs with me in it. Book specs show up to 3180 lbs. in 1974. Your car is much lighter so mileage improvements would be better.
So you may be thinking how did I go from an average 22 mpg to over 25 mpg?
After many discussions with "Doc" Doug Dutra, he convinced me to forget about horsepower and work on fine tuning and building torque.
My rear end ratio is 2.76, a 489 case with a Sure-Grip, 8 3/4 rear end. I tore up (4) 7.25" rear ends on my daily commutes, 130 mpg a day over about a 5 years. I run 26" tall tires, P215 -70R 14" tires. So torque is a must with a heavy car and high gear ratio.
Doc felt the 1920 Holley Economaster R7585, was a great base line. It has the wagon wheel style venturi for excellent air/fuel mixing. For reference: most stock 1920's are not designed this way. The 9.5 compression ratio is one of the biggest hits on developing more torque. Also longer valve seat time by running .012 and .022 helps mileage. One thing you see Doc mentioning a lot is a free flowing exhaust. I have tested many onfigurations and I really like running 2.5" pipe all the way but, you create more noise, so I run a 18" FM HP2 dual cone muffler up front into a FM 70 in the stock location. This system is super quiet and a pleasure on trips and is very free flowing. I drive with a vacuum gauge and from 2200 to 2500 rpm the engine has massive amounts of power and excellent vacuum, 16" to 22".
Ok, key things to do improve mileage.....after the engine rebuild
1. #56 jet in the Economaster. Rebuild every 5 years do to the ethanol in the gas. Very spunky set up.
2. Dual 10" electric fans. They rarely ever come on and reduce engine drag, and improve intake manifold heating.
3. Heat shield so the intake warms up quicker and keeps the carb cooler, no carb heat soak.
4. Air dam under the car for improve air flow into the radiator.
5. Distributor recurve, this is a big one. I really enjoyed that project. Big mileage improvement and it makes the torque hit very hard.
6. Cold air intake for the carb.
7. Aluminum single barrel intake. This intake is awesome.....warms up very quick for great atomization over a cast iron intake. This plus the intake heat shield are a must. I making a new one which is a little thicker and much bigger so it covers the entire intake.
8. Hotter spark, Pertronix 60,000 volt 3 ohm coil, no ballast resistor, and 8mm wires and MO-3000 rotor with .045 spark plug gap.
9. Mobile 1 Synthetic oil
10. SL6Dan's fuel line mod.
These improvements helped me get to 25+ mpg. My body style is very boxy, 74 Swinger. Doc said to drop the nose a little and that would help. So I have about a 2.5" rake with the new rear 6 leaf springs, plus the air dam. A Duster body style will get much better mileage up to 30 MPG. I have been in the car that got that mileage. It had a 4 speed and overdrive and a crappy Holley 1945 like my car came with.....I could never get over 24 mpg with that carb.
Hopefully my experiences will help you. As you see it is a combination of improvements to keep the engine happy and running at peak efficiency.
Click on the red link below my name to view some pictures of changes over the years.
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