The 2007 & later emission compliant tractors do require more maintenance than earlier models due to having to change/clean the DPF but they are not as costly as retrofitting earlier models. The retrofit usually costs more than the tractor is worth & the retrofit kits are not reliable. The company is keeping all of the 07 & later compliant units because many states/cities require that trucks operating there meet the standards & retrofitting older units is not feasible. Homesick is right about replacing the older units with nearly a million miles & there have been no new units purchased this year to allow the older road units to be moved to city operations. If the company wanted older pre 07 compliant equipment they would keep what they have.