How long does an engine last in a diesel highway tractor?

Typically, a well maintained semi tractor engine will last between 800,000 and 1.2 million miles. This all depends on proper maintenance!! I have operated a semi tow truck for the last 11 years, and I've seen it go both ways. Neglect will kill a big diesel fast!! But for the most part, any over the road tractor, with less then 400,000 miles is still relatively young. Neglected trucks can have engine failure as soon as 500,000.