Different things, surely? Seconds to half mile is a measure of (average) acceleration; we’re starting from a standstill. To get the speed after x seconds you ne

Different things, surely? Seconds to half mile is a measure of (average) acceleration; we’re starting from a standstill. To get the speed after x seconds you need to know the acceleration curve and integrate; knowing the average acceleration is not enough. Kph or mph are measures of (instantaneous or sustained or average) speed and do not tell anything about acceleration.