stock for stock???? the MT will be faster depending on your skill in driving.
Modified???? the AT can be much faster than the manual hence the reason that most transmissions in serious drag racing competition are CO2 shifted automatics.
Also, Autos are faster off the line especially in heavier cars due to the torque converter which for the first few feet of travel multiplies available torque by as much as two times. so a car with 400lbsft will launch with up to 800lbsft of force. this quickly goes away but is usefull non the less.
the type of transmission you choose depends mostly on what kind of driving you want to do. My personal view is...
Drag racing - AT rules, unquestionably
Road racing - MT is the only logical choice, at least if you want to win
Daily driving - AT is easier, especially in traffic. but the MT is a helluva lot more fun.
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Chevrolet Camaro - I enjoy beating the hell out of people
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