What's the Difference?

What's the difference in fuel mileage between a 1.0L engine and the 1.3L? Also would it be very difficult to do a rebuild from the smaller one to the larger one?

In the GEN2 cars (89-94) about 5 mpg. Of course, that's the base engine with a manual, XFi adds five, automatic cuts five to ten.

In the heavier, larger GEN3 cars (95-01) it's about 3 mpg as the three cylinder is overmatched by the cars' weight. I've had two 3/5's which both got 43 mpg in my specific driving mix, and two 4/5's which both got 40 mpg.