Just to share my experience.
I upgraded my exhaust system from a stock 2.25" which went down to 1 7/8 by the tail pipe, to a complete 3" mandrel bend system. I also upgraded the swing valve to 3" from 2.25". My mileage went from about 26MPG to 30-32MPG.
This is on a 1988 Chrysler 2.2 Intercooled Turbo.