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Anywhere between 10 and 12...but that isn't near stock (3.80:1 rear, 245/50-16, 11.2:1 comp, 650DP, custom cam, etc.).
Stock, I would average 18mpg easily with the 302-2v and 3sp with the 3.25:1 rear. With the 3.00:1 rear is was a little better with 25 mpg on the highway.
I have found with my 65 289 with 280 gears, that if I cruise on the freeway at or below 70 MPH, I get over 20. But, if I cruise at a consistent 80 MPH, I get about 17. My car just seems to cruise better at 80 than 70 though, so I sacrifice the 3 or 4 MPG. If you have a 4V, you will actually get better MPG on the highway than with a 2V. Just don't kick in the secondaries.