As for MPG increase, it depends on the particular vehicle and operator.
I Installed an underdrive crankshaft pulley/harmonic damper from ASP on my 4.0 liter Mustang in May, 2015. It's a 25% underdrive pulley. ASP made a remarkable effort to keep the damping inertia close to that of the OEM pulley/damper. It's a redesigned product; not just a modified OEM pulley/damper.
The first week after I installed this pulley, I got several alarms from my UltraGauge indicating "pending trouble code". The codes were for misfires - probably due to the lower rotating mass of the new pulley/damper. (It's detected by the crankshaft position sensor.) It hasn't happened since then. I've had no charging or cooling issues, and my butt-ometer feels a bit more power.
Prior to installing this pulley, I had 6 fill-ups over 32 MPG in an 8-year period, but they were all "one in a row". After I installed it, I had 8
consecutive fill-ups over 32 MPG (2 were over 33).
In my case, it gave me an average of about 3% (1 MPG) and I have the
fuel log data to prove it.
It'll pay for itself in fuel savings in about 200K miles. In the mean time, I've got bragging rights. (32+ MPG commuting with a Ford Mustang)
It worked for me, but, as with any modification,
your mileage may vary.