1xxx - Small Wheels, Long Wheelbase
2xxx - Small Wheels, Short Wheelbase
3xxx -
4xxx - Large Wheels, Long Wheelbase


Though mainly overall Arriva North Easts number scheme is still a bit messy being a hodgepog of Northumbria and North East Bus numbering originally. I believe there was plans for a complete renumbering on a number of occasions, but they either got part way though it and stopped renumbering, for example the MCW Metrobuses were planned to be numbered into the 7xxx range originally (I do recall seeing one or two with internal fleetnumbers with a 7 added to their 6xx number.

They probably change the rules again when these Volvo B8RLEs come, who knows what they're going to be numbered as. Just wish they renumbered the 15xx Streetlites into the 16xx range instead. And also split the double deckers up a bit.
74xx - Volvo B9TL
75xx - ADL E400
76xx - VDL DB300
77xx - ADL E400MMC.