500 cfm carb, will it harm my mileage?

What engine do you have???

If you've got a 2.3 or a 2.8, the answer is yes, the 500cfm carb will more than likely hurt your mileage as that caburetor is larger than what the stock camshaft and head(s) can handle. The 350cfm carb wouldn't be a bad choice for a 2.8 as long as the exhaust was also upgraded and it was properly tuned, the 2.3 would require some fabrication to fit the 350cfm Holley 2-bbl.

If your car has a 302, then the answer is maybe. If your car is in good running order, and you know what you're doing with that Holley 500cfm 2-bbl. carb, you can adjust and tune it so that you'll pick up a little performance without hurting your mileage. The 350cfm carb isn't really all that great of a choice for a 302 because the stock carburetor is pretty close to the same flow numbers and was already factory calibrated for your engine.

I had a Holley 500 CFM on a 1970 302-2V auto fastback Mustang many years ago. The car was stock with the only mod being dual exhaust. I received a very nice sit-of-the-pants boost. I never dyno'ed it as we didn't have access to them in the late 70's, but it felt like probably a 15 to 20 hp boost. I was really surprised but my mpg actually increased slightly. Using a stopwatch, my 0-60 mph time dropped about a second and a half, and I gained about 10 mpg on the top speed. It worked great for me. I played around with the jets a bit and ended up with either 71's or 73's.

Looking at old car magazines from the early 70's showed healthy increases over the standard 2bbl carbs and some were even nearly as good as 4bbl's. For me back then it was a great mod. I always thought if I ever picked up a 351-2V powered vehicle it would be a great mod. Of course back then they only cost around $59 to $79 through the JC Whitney catalog.
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