There is no way a 5.7 1996-2000 tahoe is seeing 18-19mpg unless they are running non ethanol fuel, super small street tires, its 2wd and driving 55mph a long distance. 2001-2003 Tahoe with a 5.3l wont' have the power or torque of the 5.7, but is a little easier on fuel maybe 16mpg?