Apparently the magic number is 16 miles per gallon. From the individual vehicle profiles in the Consumer Reports 2015 Auto Issue:
Chevrolet Suburban: “Beyond that, it’s pretty much your tried and true Suburban, with a 5.3-liter V8, six-speed automatic, and fuel economy that improved to a whopping 16 mpg.”
Chevrolet Tahoe: “Beyond that, fuel economy from the 5.3-liter V8 and six-speed automatic, improves to a whopping 16 mpg.”
GMC Yukon/Yukon XL: “Beyond that, fuel economy from the 5.3-liter V8 and six-speed automatic improves to a whopping 16 mpg, but the combination doesn’t feel particularly energetic.”
Beyond that, these trucks are more alike than different, but I don’t think I’ve ever seen CR test a thesaurus.