Dave, my '76 Bonnie with a 400/2 occasionally would touch 20 mpg, and often could average 17. That was averaged on the odometers 2nd time around. If you can find one, a "economy guage" (aka vacuum guage) helped me get that.
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Two 1975 455 Grandvilles &
'79 455 Trans Am
‘69 Camaro SS 396/375 (owned since ‘88)
‘67 Corvette 427/435
‘23 Lexus LS500 awd
‘20 Audi R8 & ‘25 Toyota RAV4 (wife’s cars)
‘95 Ford F250 4wd 7.3 p-stroke
& countless Jeeps & off road vehicles & motorcycles 
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