2550th Street:
http://www.google.com/maps?ll=40.28699,-89.364338&spn=0.06233,0.117073&t=h2300th Avenue:
http://www.google.com/maps?ll=39.960675,-89.160576&spn=0.06263,0.117073&t=h Both are found in the Logan County, Illinois. It appears they possibly go higher in some points but Google Maps are reporting them as CR (county road) 2600N. The surrounding counties seem to just use address numbers for route like 1000N, the same as in Salt Lake City, Utah.
The highest numbered Avenue in Arizona is
579th Avenue at the Maricopa County/La Paz County Line, it is 72 miles away from Central Avenue; while the highest numbered Street is only
228th Street a half mile away from the Maricopa/Pinal County Line, its 28.5 miles away from Central Avenue.
-- John O.