Did a flowmaster improve gas mileage?

No, a Flowmaster exhaust system generally does not improve gas mileage. In fact, it's more likely to slightly *decrease* fuel economy. While a less restrictive exhaust can improve engine breathing at higher RPMs, the resulting increase in engine noise and possibly increased engine load at lower RPMs usually outweighs any potential gains. The performance gains are generally felt more in terms of horsepower and torque, not fuel efficiency.