Just got back from our semi-annual trip to Florida. 2018 GMC 3500 Duramax SRW with 16,000 miles on it. Left for Orlando from Burlington, Ontario Canada. Drove 1305 miles without towing; averaged 70 mph; got 20.8 mpg.
Towed 16,000 lb 44ft 5th wheel back home, total towing 1,250 miles (stopped in Niagara Falls Ontario); got 10.5 mpg towing.
I am really pleased with the 2018. The additional horse power and torque was really noticeable compared to my 2011 2500 HD Duramax. It did not gear down on most uphill climbs as the 2011 did. The best mileage I got on the 2011 was 11.1 mpg towing 14,000 40 ft 5th wheel, low was 9.9 mpg with headwind.
2018 3500 Duri HD Crew, SRW