Amervo is totally correct....
talking of gear ratios, I have the high gear , its 2.56 rear, have a 72 model 400 (orig. a 2 bbl carb motor) w a 650 holley, and w Q`s help I picked up a set of 5C heads (large valves ,good flowing) had them milled to 87 cc`s, installed 1.65 rockers and kept the orig. cam, and its automatoic th350
( I decided to do this easy and cheap,w/ engine in car)

now,
compared to my buddy that has a 69 400 , and 5 spd
Richmond and I am not sure excactly on his rear end ,but its low, prob. close to 4...

he runs 4500 rpms at 70 mph, I do 3000 at 90!
Plus I beat him to 35-40 mph....first couple of yards he`s ahead , but very soon there after Im way ahead! then he doesnt see me! lol
Not sure of his gas mileage , but I get 14 at 75mph , and about 9-10 around town...