The reason why I'm only getting 6.5 MPG is because my current truck has over 500,000 miles on it and has several mechanical problems the company has chosen not to fix. They are not safety issues, but they do cut into fuel economy. The repairs would be costly and my truck is due for trade in any day now. I should be making around 7 MPG, but the inefficiancies of mechanical issues and age it's just not as good as it should be. The Caterpillar C-13 engine has continuing troubles with fuel injector actuators. My truck suffers from it as well. I've had the actuators replaced once in the last year and they are at it again As for how I can run 600 miles a day, here's how it works. I am allowed 11 hours of driving per day with 14 hours on duty. Breaks of over 30 minutes may be logged as off duty time. The log books works on 15 minute blocks of time. If I stop at noon and pull back out at 12:38 that is more than 7 and 1/2 minutes through the next time block so my departure time gets logged as 12:45 thus washing out the lost time in decelleration and accelleration back to highway speed. Perfectly legal. I have had my log books audited and compared to the GPS tracker data. My logs come up clean and legal. Yes, in 11 hours of driving I easily churn out 600 miles a day.