exhaust question

my exhaust fell off and i noticed my car is a lot slower. someone told me that the exhaust gives the car a boost. does anyone know how to calculate what size exhaust would do what at different speeds? i know my car was rel good at 30-40, and a bigger exhaust hhelps the car at higher speed. is there any way to calculate what exhausts do?

There have already been numerous and extensive threads on this subject. I suggest using the search option.