283 averaged 13.2 city, 16.1 highway
327 averaged 13.2 city, 16.1 highway (identical but it needed premium)
396 averaged 12.6 city, 14.9 highway.
The figures are U.S gallons (obviously) and are averages based on reader reported mileage.
The 283 ranged from 8 to 18 mpg in the city, 10 to 21 on the highway.
Comparing the 283 & 327, it appears that the 327 is efficient and doesn't have to work too hard. There is also no mention of transmission, but the Powerglide was most often specified and it did your mileage no favors.
6.2% of the readers had the 3-speed manual, 7.3% had the 4-speed, while 86.6% had an automatic (Powerglide / TH400 combined). No o.d. cars in the survey.
Dad ran a 250 six 3-speed manual with 3.08 rear in his '66 & his '70. I remember him saying that he averaged 19 mpg (Imperial) combined, which directly translates into 16.67 (U.S.) combined. He did see figures as high as the mid-20s (Imperial). My 67 Chevelle with a 250 Powerglide delivers 25 mpg (U.S.) on the highway (30 Imperial), but that car is about 400 or so pounds lighter than a typical B-body.
The 427 would probably deliver 10 mpg (U.S.) all the time for the average driver.
A friend's dad had a '73 Chevy with a smogger 454 that got 10 mpg city, 10 mpg highway, and 10 mpg towing a 17' house trailer. That's 10 Imperial or 8.3 U.S. 1973 = Lean mixture, retarded timing, EGR valve and a smog pump.