Well, just been and done my sums on the X21 (39KM) including all short runs between Bishop Auckland and Durham (11KM) as well as the X46 runs (16.9KM).
My BSOG total for the Monday to Friday journeys on the X21 have come in at £109.66 vs the totals for the Cobalt Clipper which come in at £120.59.
Although that is only a saving of £10.93 per day as well as the StreetDecks needing to be out on the road longer to make more money on the 310 (as well as the odd 309 and X39 trips), the fuel savings on the 310 using StreetDecks instead of heavy duty Volvo B9TL's could potentially be very good given the nature of the route. And as well as that, the total cost of ownership on the StreetDecks would reduce greatly as a result of less wear and tear once they go out of warranty. Any fuel savings made using StreetDecks over B9TLs would be put back into improving both the 309 and 310 whether it be the extra few vans for the 309 changeovers at New York (including the fuel) or extra drivers, both of which would most likely be required if the 309 and 310 no longer interworked for most of the time. Extra journeys particularly during the evenings could also be added using the money saved from BSOG and fuel economy.
The X21 on the other hand would massively benefit from B9TLs and if there was more dealer stock available at the time, then they would've been the natural step from the old 'Pronto' B7s. Furthermore, the B9TLs would be far more comfortable at some of the given speeds and challenges on the X21 and the fuel economy would largely remain unchanged, even more so if they were limited to 50MPH.
Engine size is another matter. Arriva's 14/64 plate E400s have coped well so far on the X18/X21/X22 and some of the 57 plates which didn't get in the hands of Blyth or Redcar (although starting to show their age now), have done quite well too. If the 5.1L unit had more torque and horsepower (preferably coupled with a ZF gearbox), then they'd be great for the X21 but the bottom line is that they're not naturally suited for that route.
Yes, the Angel StreetDecks (as well as the B5LH's before Dec 2017) have been scheduled during the evenings (and Sundays for the B5LH's) but that is only partial work as well as taking into account that between Durham and Newcastle, they'd be calling at nearly every single stop (and not using the A1/A1M). If they were on the X21 day in day out from early morning until 6-7pm, that is another matter.
RE: Go North East: Upcoming Service Changes v2