(23 Feb 2014, 1:29 pm)andreos1 wrote Suppose it is similar to when the heavy Nationals were replaced by the early lightweight Darts and the heavier Deltas.
Look what happened there.
The original batches of Solar's and Renowns outlasted the younger SPD's too - often working longer, harder routes as well.
What may be saved on fuel by a lightweight vehicle, may not always replicated in purchase savings over the longer term.
In regards to the SPDs, Go North East have always treated them as midi-vehicles rather than full sized single deckers (Solars/Renowns as your examples). The investment costs would have been originally considered over a 12-year period rather than a 15-year period.
Therefore the SPDs, when purchased, were designed to be withdrawn in 2011/12/13. When you look at it that way, some of the SPDs have lived for 15 years whereas most of the B10BLEs will only live for the same time period (despite the original intention of having them for three years longer).
Completely different story when you look at it from that perspective...