I would be surprised very surprised if you see any change in fuel economy. However what you will see from that kind of weight reduction in the flywheel is in your acceleration.
An interesting discussion about replacing stock flywheels with lightweight aluminum racing units such as what you did. Here is the website link =
lightened flywheels
"The are no negative effects to lightening your stock flywheel unlike replacing it with an ultra-lightweight Aluminum flywheel. Most manufacturers make the stock flywheel very heavy. This makes the engine very smooth and enables it to retain energy at part throttle cruise and up long grades for better fuel economy.
An Aluminum flywheel goes completely the other direction with almost no weight. This lets the engine rev up very quickly and allows the engine to work easier due to the reduced weight spinning around on the back of the crank. This is fine as long as you are at full throttle and wide open throttle all the time like in a Road Race or Drag Race only car. As soon as you let off the gas the engine RPM drops instantly and the car slows down. In a road car this causes surging and bucking at cruise speeds and poor driveability and clutch engagement.
A lightened stock unit on the other hand gives you the best of both worlds. It is a compromise between the overly heavy OEM unit and a Too light Racing unit. You get the benefits of both with none of the bad qualities.
Turbo cars are exceptionally critical to flywheel weight. Too light a flywheel will make the car rev faster and possibly come on boost faster but that is only half the issue. When you let off the gas to shift the car will drop off of boost just as quickly causing a poor transition when you come back on the throttle."
GH
