[NotSoFast]

I tow a 2004 2500HD Silverado with a Rancho lift kit and heavy off road tires. Usually have some cargo in the bed. With the Cat C15 engine, the milage is not very much affected by the presence of the toad. Roughly expect 5 to 5.5 mpg when operating at 70 to 75 mph and semi mountainous roads. Expect 6 to 6.5 mpg driving 60 to 65 mph on relatively flat roads such as Florida and Southeast states. Driving without toad gives me better acceleration but not noticeable improvement in mpg.