Not Ranked
I am no expert but I lot would depend on the way the 390 was set up. I had a rather wild one in my 67 mustang with a "vintage set up" big lumpy cam, 750 Holley, big heads, 12.5:1 compression, 4 speed toploader and I was lucky to get 7-8. It was almost unstreetable at idle but would run like a nashee with race fuel.
A friend of mine has the same car with a 5 speed, milder setup 390 with 10:1 and can get 16-18 mpg. I am very happy to get 10-12 in my Cobra with Roush 511 and 8 stack Fuel injection as a trade off to 600 hp and 620 of torque and it will idle all day long with out overheating.
Ron
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