Gas mileage ?

[MarkR]

That’s right. On more than one recommendation I put the Holley 570 on a fresh very mild rebuild of my 55 3sp/OD 292 back in 2004. I’m only interested in good solid dependable performance. Delta RV cam, 264° duration. Petronix, Mallory tach drive distributor, of course a 57 intake. This combo did not disappoint. Beautiful performance for 20 years from the Holley but always a slight rich fuel smell. Long story short last year it started showing signs of its age. I took the opportunity to switch to this 390 cfm Holley 4160C. I have to say Im very surprised at the performance. No obvious difference. The fuel smell and slightly rich condition are gone.
As far as mpg, I checked it this morning. Filled the tank, drove to meet my nephew at Shelby America near the strip. 42 miles round trip. Mostly stop and go with some at 60 mph. Filled up again on returning, about 3 gallons. I’m fine with that. I’ll rebuild the 570.

[paul2748]

For the Bird, I had a C-4 with basically a stock engine (312) and stock fordo rear. The 54 has a top loader 4 speed, no O/D

Paul, those 3.31 gears behind a Fordomatic? Does your 54 have overdrive?

[DynoDan’55]

Klogsdonjr, why do you have a 600 cfm carburetor on a stock engine? You might see better mileage from a carburetor with smaller primaries. MarkR recently replaced his 570 cfm Holley 4150 with a 390 cfm Holley 4160; so far, he's happy with the performance, and there's no more gas smell.

DynoDan, I know what "Teapot" carburetors are, but I'm not familiar with " Pony teapot." Did you mean "Puny"?

Pony went out of business years ago. Their modded carbs were great. Big boost in performance.

[55blacktie]

Paul, do you think the parasitic drag of the Fordomatic explains the mpg difference? I didn't check the weight of the 54, but I would think the two are within a couple hundred pounds.

[klogsdonjr]

This makes no sense, but I've noticed that if I refill at quarter to half full tank my gas mileage will be higher than if I refill towards empty. It's pretty consistent too.

The Holley 600 came off another classic, barely used but a great price. No gas smell, and it runs great. No issues and easy to dial in. I'll consider a lower one whenever this one needs replacing I guess. For now it's working great.

[55blacktie]

Are you going by the gauge or the numbers at the pump?

[ward 57]

It doesn't make sense to me either. Gas tanks are not square as in my wife's car when filled up the gauge does not move much with a full tank but when it starts getting low it moves much faster as the volume at the bottom of the tank is much smaller. Kinda like a 'V' but the milage remains constant as per the computer.

[289ace]

Where you live can also make a difference in mileage as the additives that some states/municipalities require, decreases the energy content and so mileage suffers. Ethanol has ~30% less energy per volume than gasoline so as ethanol increases (mandated in some areas) , mpg decreases. I used to live in California and traveled to Utah frequently in my truck. After fill ups in St George, my mpg always increased ~20% while in Utah going from ~14mpg to 17mpg. When I did some on line checking I found Cal gas had many additives and about 18% less energy in it than "normal" gas sold in other states.

I live in Az now and my truck gets around 17mpg on the highway but I don't drive the Tbird enough to know my mileage in it.

Klogsdonjr, could the vapor recovery nozzle shut off the fill earlier when you are filling a tank that is fuller? No clue if that makes any sense, but that might be a variable.

[klogsdonjr]

Based purely on actual calculations , not based on gas gage. Again, it makes no sense but fairly consistent.