Can't help with the mpg comparision, but, FWIW, all L5P pickups come equipped with 3.73's, including the fleet and CC models.
Generally the 6.0 (L96) pickups come with 4.10's and the GM (as opposed to Allison) transmission. Some fleet and CC model gassers are available with 3.73 or 4.10.
2018 L5P Sierra 4x4 SB Denali
Sold 2011 LML Sierra 4x4 CCSB SLT
Sold 2010 LMM Sierra 4x4 CCSB SLT
Sold 2004.5 LLY Sierra 4x4 ECLB SLE