ehhh, depends what your doing… on average they get pretty decent milage, a 3/4 ton duramax gets right around 21-23 on the highway… this was one guy i talked to. another said he gets around 27mpg highway
milage also flucautes on what kind of fuel you run… i’ve tried out “off road diesel” and got better MPG then i did with ULSD you get at the pump.