The car salesman tells you that the car can go from a stop position to 60 mph in six seconds is giving you the car’s rate of

The salesman is telling you the average magnitude of the car's acceleration.

| Acceleration | = (change in speed) / (time for the change)

| Acceleration | = (60 mi/hr) / (6 sec)

| Acceleration | =  10 miles/hr-sec

That would be 36,000 miles per hour squared,
or 0.0028 mile per second squared.