I would be cautious saying that a low MPH truck is a bad truck to buy, there are items, like exhaust brakes, and high idle feature that would keep the engine at a safe temperature for as long as one would want to run the motor. I have an air compressor and have used it on occasion for hrs, High idle, and there is really no issue other then I'm using a really over powered motor to run an air compressor but I need it so I used it. Also there are plenty of Hot shot drivers that park and sleep with the motor on for heat or AC. Would you say they have a bad motor because there Average MPH is low…
If they drove and left there motor on the entire time, averaged 60 MPH for every hr they can drive which is 60 Hrs in a 7 day period at the point of getting to 66 Hrs 5. 25 to get to 66 hrs so there breaking the law they park and shut of the motor They still wouldn't have a 35 MPH average.
Hot Shot drive example
Driving 11 hrs and then off sleeping 10 hrs per Hours of service.
Days 5. 25
Hrs on Engine 126
Millage 3,600
Miles per day Average 685. 71
Engine HRS per day Average 24. 00
Average MPH 28. 57
This is my truck real life experience
Purchased Date 10/4/2005
Today 6/5/2007
Days of ownership 609
Hrs on Engine 1,567
Millage 41,773
Miles per day Average 68. 59
Engine HRS per day Average 2. 57
Average MPH 26. 66
I would recomend the HR meter be used to verify millage and get an idea of how the truck might have been used.