With the new engines such as the Caterpillar Acert and the Cummins ISX, both over 450 hp, I would say an average of 5-6 mpg. But I've seen as good as 8 mpg and as poor as 4 mpg. It depends alot on idle time and terrain. A tractor that is pulling alot of mountain terrain and off road is likely to have lower mpg than that of a truck that is on level highway all the time.