It's going to vary from engine to engine. But I reduced cruising RPM by ~26%. MPG went up by ~5%.
Project 'nerd gear': taller tranny transplant nets +5.2% MPG - MetroMPG.com
Metro taller transmission swap thread: +5% efficiency gain (taller final drive) - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com
That's enough of a boost that I picked up another transaxle with the same taller final drive that one day (
) I will put in
the convertible. (One of these years I'll get around to fixing it up & putting it on the road).