Super Six fuel mileage

i have a '64 /6 engine with a 4 barrel carb and a 904 auto trans. my car gets aprox 18mpg street and 20 mpg highway. pretty good for a four barrel carb. my daily driver.

My friend has a 70 challenger convert with a single barrel carb and a stock /6 engine and 904 auto trans. he gets 22mpg street and 25 highway.