Remember that fuel economy "numbers" are relative values and do not necessarily directly reflect what one would experience in real world driving. I can relate that my 3.8's in a 1998 GP GT, a 2004 Impala LS, and a 2001 Buick Lesabre do indeed achieve mileage that is almost indentical to my 2001 GP SE 3.1 on the highway, not in the city driving. There are two reasons for the equivalent highway mileage: 1. The gearing, for example, in my 2001 GP SE and 2004 Impala are different. The 3.8 would turn about 2,000 RPM at 80 MPH and the 2001 3.1 turns about 2,300-2,400 RPM at the same speed. The increase of 300-400 RPM for the 3.1 at the same speed as the 3.8 L results in approximately similar gas mileage from the smaller motor versus the 3.8 at the same given speed. 2. The increase in RPM for the 3.1 motor at the same given speed probably results in similar horsepower being generated from the 3.1 versus the 3.8 at a given highway speed, thus similar mileage at a crusing spreed.
There is and has been a big difference in the city mileage that I have experienced with my current 3.1 SE versus the aforementioned 3.8 motors. Around town driving by the 3.1 motor is much better than any of the 3.8's I have owned. I have driven a total of 250,000 miles since 1998 with all four engines in total with the current 3.1 being the most at 100,000 miles.