6 banger mileage?

Hello, I'm thinking about buying a car with a good ol slant six for a daily driver. I found some old articles on the internet that says the 60's models were rated for around 30mpg. A buddy at work says a small six from this era won't get 20mpg at idle, down hill with the wind at my back?
Any of you slant six owners have any input?
Thanks....