Due to the built-in slip in a torque converter, your engine speed will be 10% higher at the same road speed with an automatic. A lockup clutch will bring the speed back down to the same as high gear in the standard trans. Rear ends for an automatic trans are generally higher-geared, to take advantage of the torque multiplication of a converter on breakaway. For instance, my 58 Chevy sedan had a 3.55:1 rear end when it came out with a 3-speed, and the standard rear end for the Powerglide that year was a 3.36:1.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!