The smaller Duramax engines get better gas mileage than the 6.6L. These small turbodiesels are found in the Canyon and Colorado. If you are looking for the most fuel-efficient 6.6L Duramax, you can find several models that achieved around 15 mpg. Choose from the 2009 GMC Sierra 3500HD, 2003 GMC Sierra 2500 HD, or the 2002 Chevy Silverado 2500HD.