4x4 or 2 wheel drive

I m buying an excursion but want to know what real world mileage I can expect. I was told by one diesel mechanic that I would get double the mileage with 4x2. I think its really neat that I can ask my question in this forum and not have to go blindly into this endeavor. Thanks. Cowmoo

you'll get about 2, maybe 3 mpg better with 2wd. In my opinion, the fuel economy gain from 2wd isn't worth NOT having 4wd