The car salesman tells you that the car can go from a stopped position to 60 miles per hour in 6 seconds. he is giving you the car's rate of

He tells you  (a speed) per (a time).  That's a rate of acceleration .

Let's find out what the acceleration is:

             (60 mile/hour) / (6 seconds)

         =    10  miles/hour-second

Weird units.  Let's convert to something easier.

           (10  mi/hr-sec) x (5280 ft/mi) x (1 hr / 3600 sec)

         =    14-2/3  ft/sec²  =  about 0.46 'G'

          about 4.47 meter/s²