Wow, an awful lot of gas vapors would have to be escaping past that gas cap to make a significant difference in gas mileage on a car that's probably only getting 15~18 MPG to begin with. Your mileage may have improved but I'd be more likely to believe it's because you're getting more used to driving the car and aren't suffering as severely from 'new Mustang happy feet'.
Get a small notebook to keep in the car and write down your odometer reading and how many gallons you put in every time you fill it up. That's the only way you're gonna know for sure what's going on with your gas mileage. I put the date too and then off to the side I can make any notes about repairs, maintenance, upgrades or if I had to add any fluids so I can look back and have a record of all that stuff or see if there's any trends going on.
Get a small notebook to keep in the car and write down your odometer reading and how many gallons you put in every time you fill it up. That's the only way you're gonna know for sure what's going on with your gas mileage. I put the date too and then off to the side I can make any notes about repairs, maintenance, upgrades or if I had to add any fluids so I can look back and have a record of all that stuff or see if there's any trends going on.